Possibly mad on it, you'll agree (5)
I believe the answer is:
admit
'agree' is the definition.
(thesaurus)
'possibly mad on it' is the wordplay.
'possibly' indicates anagramming the letters.
'on' means one lot of letters go next to another.
'mad' is an anagram of 'adm'.
'adm'+'it'='ADMIT'
'you'll' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for admit that I've seen before include "Agree to the truth" , "Confess and permit entrance" , "Confess (to something)" , "Confess - let in" , "Confess - acknowledge" .)