Revel losing head a little bit (4)
I believe the answer is:
iota
'little bit' is the definition.
(thesaurus)
'revel losing head a' is the wordplay.
'revel' becomes 'riot' (riot is a kind of revel).
'losing head' means to remove the first letter.
'riot' with its first letter removed is 'iot'.
'iot'+'a'='IOTA'
(Other definitions for iota that I've seen before include "Greek letter - minimal amount" , "Hint" , "Tiny piece" , "Just a jot, from Greek alphabet" , "little part" .)