Marine features you can see free at Spithead (5)
I believe the answer is:
reefs
'marine features you can see free at spithead' is the definition.
I can't tell whether this defines the answer.
'can see free at spithead' is the wordplay.
'can' indicates an anagram (I've seen 'canned' mean this (canned can mean drunk)).
'at' is a charade indicator (letters next to each other).
'spithead' becomes 's' (head letter of 'spit').
'free' with letters rearranged gives 'reef'.
'reef'+'s'='REEFS'
Can you help me to learn more?
(Other definitions for reefs that I've seen before include "coral banks" , "Submerged ridges of rock or coral near the surface of water" , "Groups of submerged rock" , "Ridges of rock or sand beneath sea surface" .)