Very hard putting fish into pig-pen (6)
I believe the answer is:
steely
'very hard' is the definition.
(I've seen this before)
'fish into pig-pen' is the wordplay.
'fish' becomes 'eel' (eel is a kind of fish).
'into' is an insertion indicator.
'pig pen' becomes 'sty' (I've seen this before).
'eel' placed into 'sty' is 'STEELY'.
'putting' acts as a link.
(Other definitions for steely that I've seen before include "With (cold) resolve" , "Set off" , "Coldly determined" , "Inflexibly severe" , "Obdurate" .)