Poles suffer internally - they're hammered (5)
I believe the answer is:
nails
'they're hammered' is the definition.
(nails are hammered)
'poles suffer internally' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'suffer' becomes 'ail' (ailing is a kind of suffering).
'internally' means one lot of letters goes inside another.
'ns' placed around 'ail' is 'NAILS'.
(Other definitions for nails that I've seen before include "they may provide security for the board" , "Metal fasteners" , "Fastens with spikes" , "Filed regularly" , "Horny plates of the digits" .)