Pronunciation of 一 in 一个
一 is pronounced in the first tone when it stands alone.
It is pronounced in the fourth tone when it precedes a first, second, or third tone. However, it is pronounced in the second tone when it precedes a fourth tone. 不 is a bit similar: It is also pronounced in the fourth tone when it precedes a first, second, or third tone. However, it is pronounced in the second tone when it precedes a fourth tone.

Some examples (from: http://web.mit.edu/jinzhang/www/pinyin/tones/index.html#1bu but I used simplified characters instead and corrected a tone which is incorrect on that website)


一万 (yíwàn)
(ten thousand)  
一块 (yíkuài)
(one dollar)
一半 (yíbàn)
( a half)
不贵 (búguì)
(not expensive) 
不对 (búduì)
(not correct)
不慢 (búmàn)
(not slow)
一天 (yìtiān)
(a day) 
一生 (yìshēng)
(one's whole life)
不多 (bùduō)
(not many/much)
不吃 (bùchī)
(not eat)
一年 (yìnián)
(a year)    
一群 (yìqún)
(a group of)
不难 (bùnán)
(not difficult)
不忙 (bùmáng)
(not busy)
一晚 (yìwǎn) 
(one night) 
一早 (yìzǎo)
(early in the morning)
不晚 (bùwǎn)
(not late)
不早 (bùzǎo)
(not early)


Since 个 has a fourth tone, the tone of 一 should become a second tone. Although in practice 个 is most often spoken as a neutral tone, the original tone is used for the tone sandhi.