Lacking a natural ability to eat seconds (6)
I believe the answer is:
absent
'lacking' is the definition.
(I've seen this before)
'a natural ability to eat seconds' is the wordplay.
'natural ability' becomes 'bent' (bent is a kind of natural ability).
'to eat' is an insertion indicator.
'seconds' becomes 's'.
'bent' enclosing 's' is 'bsent'.
'a'+'bsent'='ABSENT'
(Other definitions for absent that I've seen before include "Not attending" , "AWOL" , "Not present or focused" , "Missing or dreamy" , "lack of attention" .)