Gastropods which exhausted deer (6)
I believe the answer is:
whelks
'gastropods' is the definition.
(whelk is a kind of gastropod)
'which exhausted deer' is the wordplay.
'exhausted' says to hollow out the word (remove centre letters) (the word is emptied out or exhausted).
'deer' becomes 'elks' (I've seen this before).
'which' with its middle removed is 'wh'.
'wh'+'elks'='WHELKS'
(Other definitions for whelks that I've seen before include "Kind of sea snails used as food" , "Marine snails" , "Seasnails" .)