Start to trim headgear, the one over there (4)
I believe the answer is:
that
'the one over there' is the definition.
(I've seen this before)
'start to trim headgear' is the wordplay.
'start to' says to take the initial letters.
'headgear' becomes 'hat' (hat is a kind of headgear).
The initial letter of 'trim' is 't'.
't'+'hat'='THAT'
(Other definitions for that that I've seen before include "See 5" , "Take ..., English pop band" , "Not 8 Ac but the other" , "Yon" , "Just like --; -- is to say" .)