Let in tiny boy coming back after short notice (5)
I believe the answer is:
admit
'let in' is the definition.
(synonyms)
'tiny boy coming back after short notice' is the wordplay.
'tiny boy' becomes 'Tim' (A Christmas Carol character Tiny Tim).
'coming back' shows that the letters should be reversed in order.
'after' means one lot of letters go next to another.
'short notice' becomes 'ad' (both can mean an advertisement. I am not sure about the 'short' bit.).
'tim' reversed gives 'mit'.
'mit' put after 'ad' is 'ADMIT'.
Can you help me to learn more?
(Other definitions for admit that I've seen before include "Usher in" , "Confess (to something)" , "permit to come in" , "Confess; allow in" , "Come clean" .)