Difficult word requested of sitters bringing bad luck (4,6)
I believe the answer is:
hard cheese
'bad luck' is the definition.
(I've seen this before)
'difficult word requested of sitters' is the wordplay.
'difficult' becomes 'hard' (associated in meaning).
'word requested of sitters' becomes 'cheese' (people sitting for a photograph say 'cheese').
'hard'+'cheese'='HARD CHEESE'
'bringing' acts as a link.
(Other definitions for hard cheese that I've seen before include "Bad luck! (Grate it!)" , "commiserating words" , "Tough luck (informal)" , "Expression of sympathy" , "Pecorino" .)