Left with pain, weaken (8)
I believe the answer is:
languish
'weaken' is the definition.
(languishing is a kind of weakening)
'left with pain' is the wordplay.
'left' becomes 'L' (common abbreviation).
'with' says to put letters next to each other.
'pain' becomes 'anguish' (synonyms).
'l'+'anguish'='LANGUISH'
(Other definitions for languish that I've seen before include "lose strength" , "Lose or lack vitality" , "Pine, waste away" , "Suffer neglect" , "Long" .)