It's wrong to start messing with him! (3)
I believe the answer is:
tim
'him' is the definition.
(short for Timothy)
'it's wrong to start messing' is the wordplay.
'wrong' indicates an anagram.
'to start' suggests taking the first letters.
The initial letter of 'messing' is 'm'.
'it' anagrammed gives 'ti'.
'ti'+'m'='TIM'
'with' acts as a link.
(Other definitions for tim that I've seen before include "Tiny . . . is a loveable Dickens boy" , "Adjoin" , "Tiny . . . is a lovable small boy from Dickens" , "Tiny -, Dickens character (A_Christmas Carol)" , "Tiny Dickens boy from 'A Christmas Carol'" .)