Mum under small blanket (7)
I believe the answer is:
smother
'blanket' is the definition.
(I've seen this in another clue)
'mum under small' is the wordplay.
'mum' becomes 'mother' (I've seen this before).
'under' says to put letters next to each other (in a down clue, some letters go below others).
'small' becomes 's' (abbreviation - e.g. clothes size).
'mother' after 's' is 'SMOTHER'.
(Other definitions for smother that I've seen before include "Prevent from breathing" , "Conceal" , "blanket" , "Muffle" , "Put out" .)