Poor stag upset by devouring first mollusc (9)
I believe the answer is:
gastropod
'mollusc' is the definition.
(I know that gastropod is a type of mollusc)
'poor stag upset by devouring first' is the wordplay.
'upset by' is an anagram indicator.
'devouring first' becomes 'd' (1st letter of 'devouring').
'poor'+'stag'='poorstag'
'poorstag' with letters rearranged gives 'gastropo'.
'gastropo'+'d'='GASTROPOD'
(Other definitions for gastropod that I've seen before include "Mollusc such as snail or limpet" , "Little creature" , "Eg, a snail or slug" , "Escargot" , "slug, perhaps" .)