Lowlife heading for major argument, but held back (4)
I believe the answer is:
worm
'lowlife' is the definition.
(I've seen this in another clue)
'heading for major argument but held back' is the wordplay.
'heading for' suggests taking the first letters.
'argument' becomes 'row' (to row is to argue).
'but held back' shows that the letters should be reversed in order.
The initial letter of 'major' is 'm'.
'm'+'row'='mrow'
'mrow' written backwards gives 'WORM'.
(Other definitions for worm that I've seen before include "mean creature" , "Screw thread; maggot" , "Insect larva" , "Wriggle" , "Long slender invertebrate" .)