Sees vessels at Spithead (5)
I believe the answer is:
spots
'sees' is the definition.
(spot can mean to notice or see)
'vessels at spithead' is the wordplay.
'vessels' becomes 'pots' (pot is a kind of vessel).
'at' means one lot of letters go next to another (I've seen this in other clues).
'spithead' becomes 's' (head letter of 'spit').
'pots' put after 's' is 'SPOTS'.
(Other definitions for spots that I've seen before include "Result of outbreak, possibly" , "Espies the blobs?" , "Picks out" , "Blemishes" , "Catches sight of" .)