Backfiring Cupid's arrow essentially gets more painful (5)
I believe the answer is:
sorer
'more painful' is the definition.
('sore' can be similar in meaning to 'painful')
'backfiring cupid's arrow essentially' is the wordplay.
'backfiring' shows that the letters should be reversed in order.
'cupid' becomes 'eros' (I've seen this before).
'essentially' says to take the centre.
The middle of 'arrow' is 'r'.
'eros' backwards is 'sore'.
'sore'+'r'='SORER'
'gets' acts as a link.
(Other definitions for sorer that I've seen before include "more upset" , "Comparatively vexed" , "More painful or vexed" , "more resentful" .)