Beater back from reserve overrun by deer (5)
I believe the answer is:
heart
'beater' is the definition.
(organ which beats)
'back from reserve overrun by deer' is the wordplay.
'back from' says to take the final letters (the letter at the back of the word).
'overrun by' is an insertion indicator (I've seen this in other clues).
'deer' becomes 'hart' (hart is a kind of deer).
The last letter of 'reserve' is 'e'.
'e' placed within 'hart' is 'HEART'.
(Other definitions for heart that I've seen before include "Kernel" , "It beats" , "one in suit" , "one needs to beat" , "the name of a prison in Midlothian!" .)