Boy underneath smooth Conservative in ship (8)
I believe the answer is:
ironclad
'ship' is the definition.
(19th century armoured warship)
'boy underneath smooth conservative' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'underneath' says to put letters next to each other (in a down clue, some letters go under others).
'smooth' becomes 'iron' (to smooth down with an iron).
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'iron'+'c'='ironc'
'lad' after 'ironc' is 'IRONCLAD'.
'in' is the link.
(Other definitions for ironclad that I've seen before include "Well armoured - perhaps an alibi?" , "Inflexible" , "well-protected" , "Protected as by armour - say an alibi" , "Unassailable" .)