I think it's KEEP STEP where the military cap is KEPI (but shorter ie: KEP) and the sculptor could be EPSTEIN. Not enjoying this crossword one bit today lol !!
The answer is 'keep step'. The sculptor starts off being Epstein, but does not get completed. the military cap is a 'kepi', but only mainly i.e. 'kep' - then dropped in on by the unfinished Epstein!