Flat helmet holding tool (5)
I believe the answer is:
lathe
'tool' is the definition.
(thesaurus)
'flat helmet holding' is the wordplay.
'holding' says the answer is hidden in the clue.
'LATHE' can be found hidden inside 'flat helmet'.
(Other definitions for lathe that I've seen before include "Turner's machine" , "Turner's tool" , "piece of machinery" , "Machine tool for shaping metal or wood" , "Rotating machine tool" .)