On reflection, regret being transfixed by old guy's charm (7)
I believe the answer is:
enamour
'charm' is the definition.
(synonyms)
'on reflection regret being transfixed by old guy's' is the wordplay.
'on reflection' says the letters should be written in reverse.
'regret' becomes 'rue' (to rue something is to regret it).
'being transfixed by' is an insertion indicator.
'old' becomes 'o' (abbreviation).
'guy' becomes 'man' (guy can informally mean a man).
'o'+'man'='oman'
'rue' going around 'oman' is 'ruomane'.
'ruomane' back-to-front is 'ENAMOUR'.
(Other definitions for enamour that I've seen before include "Enchant" , "Fill with love" , "Inspire with love; charm" , "sweep one off one's feet" , "Our name (anag)" .)