Horse backed in front of good crowd (4)
I believe the answer is:
gang
'crowd' is the definition.
(I've seen this before)
'horse backed in front of good' is the wordplay.
'horse' becomes 'nag' (nag is a kind of horse).
'backed' shows that the letters should be reversed in order.
'in front of' suggests taking the first letters.
The first letter of 'good' is 'g'.
'nag' written backwards gives 'gan'.
'gan'+'g'='GANG'
(Other definitions for gang that I've seen before include "Association of criminals" , "Criminal mob" , "Working group or mob of hoodlums" , "Group (crazy or criminal?)" , "criminal family" .)