Hefty? Pounds lighter, though not at first (5)
I believe the answer is:
large
'hefty?' is the definition.
(I've seen this in another clue)
'pounds lighter though not at first' is the wordplay.
'pounds' becomes 'l' (abbreviation e.g. Lsd - pounds, shillings, pence).
'lighter' becomes 'barge' (synonyms).
'though not at first' means to remove the first letter (the first letter is removed).
'barge' with its initial letter removed is 'arge'.
'l'+'arge'='LARGE'
(Other definitions for large that I've seen before include "Copious" , "Great being regal" , "Above average in size, number, etc." , "Of considerable size or extent" , "Of great size" .)