Contrary mastiff urgently fitted with collar (4)
I believe the answer is:
ruff
'collar' is the definition.
(I know that ruff is a type of collar)
'contrary mastiff urgently fitted with' is the wordplay.
'contrary' shows that the letters should be reversed in order.
'with' indicates the answer is hidden within the clue.
'ffur' is hidden within 'mastiff urgently fitted'.
'ffur' in reverse letter order is 'RUFF'.
(Other definitions for ruff that I've seen before include "Elizabethan or Jacobean neck adornment" , "Frilly collar" , "Fish; neck frill" , "Male sandpiper" , "Starched neck frill" .)