Portly old boy firstly fails, see (5)
I believe the answer is:
obese
'portly' is the definition.
(I've seen this before)
'old boy firstly fails see' is the wordplay.
'firstly' says to take the initial letters.
'fails' is an anagram indicator (I've seen 'failed' mean this).
The first letters of 'old boy' is 'ob'.
'see' with letters rearranged gives 'ese'.
'ob'+'ese'='OBESE'
(Other definitions for obese that I've seen before include "Stout" , "Clinically overweight" , "Needing to lose weight" , "Unusually fat" , "Extremely fat" .)