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