Smooth youth rounds cape in armoured ship (8)
I believe the answer is:
ironclad
'armoured ship' is the definition.
(ship covered in iron)
'smooth youth rounds cape' is the wordplay.
'smooth' becomes 'iron' (to smooth down with an iron).
'youth' becomes 'lad' (I've seen this in another clue).
'rounds' is an insertion indicator.
'cape' becomes 'c' (abbreviation used on maps).
'iron'+'lad'='ironlad'
'ironlad' placed around 'c' is 'IRONCLAD'.
'in' acts as a link.
(Other definitions for ironclad that I've seen before include "Old armoured warship" , "Protected as by armour - say an alibi" , "Unassailable" , "C 19 armoured warship" , "Protected warship" .)