Flirt is more sensible, old fellow having been taken in (9)

I believe the answer is:
womaniser
'flirt' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'is more sensible old fellow having been taken in' is the wordplay.
I cannot really understand how this works, but
'is' is within the answer.
'more' could be 'm' and 'm' is found within the answer.
'old' could be 'o' (abbreviation) and 'o' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for womaniser that I've seen before include "Ladies' man" , "Lady-killer" , "Man amorously pursuing females" , "Wolf" , "A rake" .)
