Poles have to suffer - they're hammered! (5)
I believe the answer is:
nails
'they're hammered' is the definition.
(nails are hammered)
'poles have to suffer' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'have' is an insertion indicator.
'to suffer' becomes 'ail' (ailing is a kind of suffering).
'ns' enclosing 'ail' is 'NAILS'.
(Other definitions for nails that I've seen before include "Fastens with spikes" , "Horny plates and pointed pins" , "Horny plates of the digits" , "Slain with metal pins" , "Filed regularly" .)