Case statement query
Your example query appears to be incomplete (e.g. your aliases t and cl don't correspond to anything in the example). Ignoring that for a second and using your example query, this is probably what you're after:
SELECT t.status_id AS status_id, cl.category_code + '-' + s.status_name AS status_type
FROM SomeTable AS t
LEFT JOIN firm_return AS s
    ON t.firm_id = s.firm_id
    AND s.enabled = 'D'
... -- The rest of your query logic here (e.g. the table for the cl alias)

If you care about NULLS and want to handle them with a default value then you can use the ISNULL() function around the logic for your status_type column like so: ISNULL(cl.category_code + '-' + s.status_name, 'SomeDefaultValueGoesHere') AS status_type