The mariner did have an anorak (4)
I believe the answer is:
nerd
'an anorak' is the definition.
(I've seen this before)
'the mariner did have' is the wordplay.
'have' indicates the answer is hidden within the clue.
'NERD' is hidden within 'mariner did'.
(Other definitions for nerd that I've seen before include "Boring, trivia-obsessed type" , "Flag on poles; wide (headline)" , "Boring person" , "Computer geek?" , "In slang, a foolish or uninteresting person" .)