Sort of job that's popular in firm? (8)
I believe the answer is:
sinecure
'sort of job' is the definition.
'popular in firm?' is the wordplay.
'popular' becomes 'in'.
'in' indicates putting letters inside.
'firm?' becomes 'secure' ('secure' can be similar in meaning to 'firm'**).
'in' put within 'secure' is 'SINECURE'.
'that's' acts as a link.
(Other definitions for sinecure that I've seen before include "An office without work." , "Insecure in easy job" , "Office or job which involvesno work" , "Undemanding job" , "position needing little effort" .)