Left with severe pain, become weak (8)
I believe the answer is:
languish
'become weak' is the definition.
(languishing is a kind of becoming weak)
'left with severe pain' is the wordplay.
'left' becomes 'l' (common abbreviation).
'with' is a charade indicator (letters next to each other).
'severe pain' becomes 'anguish' (I've seen this before).
'l'+'anguish'='LANGUISH'
(Other definitions for languish that I've seen before include "Become discouraged" , "go into decline" , "Flag" , "Droop" , "Pine, waste away" .)