Man in coalfield (3)
I believe the answer is:
alf
'man' is the definition.
(short for Alfred)
'in coalfield' is the wordplay.
'in' says the answer is hidden in the clue.
'ALF' is hidden within 'coalfield'.
(Other definitions for alf that I've seen before include "Boy" , "Mr Garnett" , "- Ramsey, England manager in nineteen sixty six" , "Sir - Ramsey, England footballer and manager" , "Garnett or Ramsey?" .)