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' says to put letters next to each other.
'lead in' suggests taking the first letters.
The first 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" .)