Collar half of hooligans (4)
I believe the answer is:
ruff
'collar' is the definition.
(ruff is a kind of collar)
'half of hooligans' is the wordplay.
'half of' indicates taking half.
'hooligans' becomes 'ruffians' ('ruffian' can be a synonym of 'hooligan').
'ruffians' cut in half is 'RUFF'.
(Other definitions for ruff that I've seen before include "Elizabethan or Jacobean neck adornment" , "Trump; male shore bird" , "Starched frill; fish" , "Type of collar" , "Starched neck frill" .)