Kid a fellow, that's all (5)
I believe the answer is:
total
'all' is the definition.
(thesaurus)
'kid a fellow' is the wordplay.
'kid' becomes 'tot' (tot is a kind of kid).
'a fellow' becomes 'al' (I've seen this in other clues).
'tot'+'al'='TOTAL'
'that's' is the link.
(Other definitions for total that I've seen before include "Entire sum" , "Whole amount" , "Complete sum" , "Final tally" , "Complete; add up" .)