"A powerful feller!" Bond observed (8)
I believe the answer is:
chainsaw
'a powerful feller' is the definition.
(something which might fell trees)
'bond observed' is the wordplay.
'bond' becomes 'chain' (chain is a kind of bond).
'observed' becomes 'saw' (seen).
'chain'+'saw'='CHAINSAW'
(Other definitions for chainsaw that I've seen before include "powered device" , "Texas massacre tool" , "Power tool" , "Powered cutter" , "Timber cutter" .)