Senior fool definitely has it (3)
I believe the answer is:
old
'senior' is the definition.
('old' can be similar in meaning to 'senior')
'fool definitely has it' is the wordplay.
'has it' indicates a hidden word.
'OLD' is hidden in the letters of 'fool definitely'.
(Other definitions for old that I've seen before include "Advanced in years" , "Elderly" , "Ancient" , "No longer young" , "Former; aged" .)