Number 1 single by Band Aid (4)
I believe the answer is:
help
'aid' is the definition.
(synonyms)
'number 1 single by band' is the wordplay.
I cannot quite see how this works, but
'number' could be 'l' (L is an example) and 'l' is located in the answer.
The remaining letters 'hep' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for help that I've seen before include "one by the Beatles" , "Come to the aid of" , "Support" , "Guide" , "Beatles' album; assist" .)