Fly low nerve required (10)
I believe the answer is:
bluebottle
'fly' is the definition.
(bluebottle is a kind of fly)
'low nerve required' is the wordplay.
'low' becomes 'blue' (both can mean unhappy).
'nerve' becomes 'bottle' (I've seen this before**).
'required' means one lot of letters go next to another.
'blue'+'bottle'='BLUEBOTTLE'
(Other definitions for bluebottle that I've seen before include "An insect; Goon character" , "Blowfly - policeman" , "Fly - policeman" , "Large fly -- wild cornflower" , "member of force" .)