Unknown depth by ship off shore (4,5)
I believe the answer is:
dark horse
'unknown' is the definition.
(I've seen this before)
'depth by ship off shore' is the wordplay.
'depth' becomes 'd' (abbreviation in geometry).
'by' means one lot of letters go next to another.
'ship' becomes 'ark' (as in Noah's ark).
'off' is an anagram indicator.
'shore' with letters rearranged gives 'horse'.
'd'+'ark'+'horse'='DARK HORSE'
(Other definitions for dark horse that I've seen before include "Little-known competitor like shadowy equine" , "Little is known of its properties" , "Unexpected (possible) winner" , "Competitor of unknown capabilities" , "Competitor whose abilities are unknown" .)