Sailor must keep one area for headgear (5)
I believe the answer is:
tiara
'headgear' is the definition.
(tiara is a kind of headgear)
'sailor must keep one area' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'must keep' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'area' becomes 'a' (maths abbreviation).
'tar' placed around 'i' is 'tiar'.
'tiar'+'a'='TIARA'
'for' acts as a link.
(Other definitions for tiara that I've seen before include "Decorative headgear" , "Small crown" , "Jewelled head-ornament" , "Jewelled, ornamental headgear" , "Jewelled head wear" .)