Can't stand, but the hard seat is awful (5)
I believe the answer is:
hates
'can't stand' is the definition.
(I've seen this in another clue)
'the hard seat is awful' is the wordplay.
'the hard' becomes 'h' (abbreviation used in pencil classifications).
'is awful' indicates anagramming the letters.
'seat' is an anagram of 'ates'.
'h'+'ates'='HATES'
'but' is the link.
(Other definitions for hates that I've seen before include "can't stand" , "Abhors" , "Despises" , "passionate feelings" , "Loathes" .)