Caller does not have the item for part of helmet (5)
I believe the answer is:
visor
'part of helmet' is the definition.
('visor' is part of 'helmet')
'caller does not have the item' is the wordplay.
'caller' becomes 'visitor' (I've seen this before).
'does not have' suggests deleting specific letters.
'the item' becomes 'it' (vague term for specific object).
'visitor' with 'it' removed is 'VISOR'.
'for' acts as a link.
(Other definitions for visor that I've seen before include "Part of an armour helmet" , "piece of armour" , "Eye shield" , "Face-protector" , "Transparent flap on a helmet" .)