It's a small thing to put on the song Cockney chap's forgotten (5)
I believe the answer is:
thong
'it's a small thing to put' is the definition.
I can't tell whether this defines the answer.
'the song cockney chap's forgotten' is the wordplay.
'cockney chap's' becomes ''es' ('hes' said with a cockney accent, dropping the 'H').
'forgotten' suggests deleting specific letters.
'the'+'song'='thesong'
'thesong' with 'es' removed is 'THONG'.
'on' acts as a link.
Can you help me to learn more?
(Other definitions for thong that I've seen before include "Pants" , "Skimpy garment" , "Insubstantial underwear" , "Strap - strip" , "Skimpy underwear" .)