It restrains worker, perhaps a seasonal deliveryman (5)
I believe the answer is:
santa
'a seasonal deliveryman' is the definition.
The definition suggests a singular noun which matches the answer.
'it restrains worker perhaps' is the wordplay.
'it' becomes 'sa' (SA can mean sex appeal, as in 'it').
'restrains' is an insertion indicator.
'worker perhaps' becomes 'ant' (worker is a type of ant).
'sa' enclosing 'ant' is 'SANTA'.
Can you help me to learn more?
(Other definitions for santa that I've seen before include "patron at Christmas" , "The present chap" , "Man with sack" , "Dad in a red and white outfit?" , "He comes down chimney" .)