It's a blow when the little devil has to work (6)
I believe the answer is:
impact
'it's a blow' is the definition.
(thesaurus)
'the little devil has to work' is the wordplay.
'the little devil' becomes 'imp' (I've seen this before).
'has' says to put letters next to each other.
'to work' becomes 'act' ('act' can be a synonym of 'work'**).
'imp'+'act'='IMPACT'
'when' is the link.
(Other definitions for impact that I've seen before include "Forceful striking" , "Marked effect; hit" , "Repercussion" , "Bear upon" , "Force of colllision or blow" .)