Reading of 匙 meaning “key” (as in 钥匙)
匙 http://www.cantonese.sheik.co.uk/dictionary/characters/371/
[粵] ci4 | [國] chi2 : spoon
[粵] si4 | [國] shi5 : key

When '匙' means 'spoon', it is read as /chi2/ in Mandarin
When '匙' means 'key', it is read as /shi5/ in Mandarin
It doesn't matter it is by itself or in a compound word
There are other examples of single character that have more than one meaning and pronounced differently for each meaning.  樂 is one of them. When 樂 means 'joy' it is read as /le4/; When 樂 means 'music' it is read as /yue4/