Reflect, as young person runs for navy (6)
I believe the answer is:
mirror
'reflect' is the definition.
(mirroring is a kind of reflecting)
'young person runs for navy' is the wordplay.
I cannot really see how this works, but
'runs' could be 'rr' (I've seen this in other clues) and 'rr' is located in the answer.
This explanation may well be incorrect...
'as' is the link.
(Other definitions for mirror that I've seen before include "Depict" , "which reflects what you want to see" , "Something studied constantly by the vain?" , "1 [UNLUCKY FOR SOME]" , "Looking-glass" .)