Part of helmet: caller drops it (5)
I believe the answer is:
visor
'part of helmet' is the definition.
('visor' is part of 'helmet')
'caller drops it' is the wordplay.
'caller' becomes 'visitor' (I've seen this before).
'drops' suggests deleting specific letters.
'visitor' with 'it' taken out is 'VISOR'.
(Other definitions for visor that I've seen before include "Eye shield" , "Eye protector" , "One protecting" , "Face-armour" , "Face-protector" .)