Pet sent back small drink (4)
I believe the answer is:
puss
'pet' is the definition.
(I know that puss is a type of pet)
'sent back small drink' is the wordplay.
'sent back' says the letters should be written backwards.
'small' becomes 's' (abbreviation - e.g. clothes size).
'drink' becomes 'sup' (to sup is to drink).
's'+'sup'='ssup'
'ssup' in reverse letter order is 'PUSS'.
(Other definitions for puss that I've seen before include "It might be a tortoiseshell" , "Feline" , "clock" , "Pet" , "mug" .)