Attacks endlessly hard to repulse (4)
I believe the answer is:
fits
'attacks' is the definition.
(fit is a kind of attack)
'endlessly hard to repulse' is the wordplay.
'endlessly' means to remove the last letter.
'hard' becomes 'stiff' ('stiff' can be similar in meaning to 'hard').
'to repulse' is a reversal indicator ('repulse' can mean to send backwards).
'stiff' with its last letter taken away is 'stif'.
'stif' in reverse letter order is 'FITS'.
(Other definitions for fits that I've seen before include "Prepares" , "Seizures" , "Paroxysms" , "Is of the right shape and size" , "feels comfortable" .)