Left with pain, grow weak (8)
I believe the answer is:
languish
'grow weak' is the definition.
(languish can mean to weaken or wither)
'left with pain' is the wordplay.
'left' becomes 'l' (common abbreviation).
'with' is a charade indicator (letters next to each other).
'pain' becomes 'anguish' (anguish is pain or suffering).
'l'+'anguish'='LANGUISH'
(Other definitions for languish that I've seen before include "Pine, waste away" , "Haul sign away - lack vitality" , "Wilt" , "Lose or lack vitality" , "Flag" .)