Second fiddle? That's a difficult question (7)
I believe the answer is:
stinker
'a difficult question' is the definition.
'second fiddle?' is the wordplay.
'second' becomes 's' (common abbreviation).
'fiddle?' becomes 'tinker' (I've seen this before).
's'+'tinker'='STINKER'
'that's' is the link.
(Other definitions for stinker that I've seen before include "Difficult problem" , "Terrible job" , "no easy thing" , "Very hard task" , "Unpleasant person - difficult task" .)