Fat dog on lead in field (4)
I believe the answer is:
flab
'fat' is the definition.
(flab is a kind of fat)
'dog on lead in field' is the wordplay.
'dog' becomes 'lab' (short for labrador).
'on' is a charade indicator (letters next to each other).
'lead in' indicates taking the first letters.
The initial letter of 'field' is 'f'.
'lab' after 'f' is 'FLAB'.
(Other definitions for flab that I've seen before include "Unwanted body fat" , "spare tyre?" , "Soft loose flesh" , "Loose body fat" , "Informally, unsightly fat" .)