Odd being surplus to requirements, as seaman may be (13)
I believe the answer is:
extraordinary
'odd' is the definition.
(similar in meaning)
'being surplus to requirements as seaman may be' is the wordplay.
I cannot quite understand how this works, but
'being' could be 'ordinary' (ordinary is a kind of being) and 'ordinary' is found within the answer.
'surplus' could be 'extra' (synonyms) and 'extra' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for extraordinary that I've seen before include "Phenomenal" , "Exceptional" , "Highly unusual" , "Remarkable - abnormal" .)