Enlarged garment for king to wear (5)
I believe the answer is:
grown
'enlarged' is the definition.
(grown bigger)
'garment for king to wear' is the wordplay.
'garment for' becomes 'gown' (I can't justify this - if you can you should give a lot more credence to this answer).
'king' becomes 'R' (abbreviation of Latin rex).
'to wear' is an insertion indicator (to wear something is to be covered up by it).
'gown' going around 'r' is 'GROWN'.
(Other definitions for grown that I've seen before include "increased in stature?" , "Increased; developed" , "Cultivated" , "Fully developed" , "Matured" .)