For central sailor left half is redoubtable (10)
I believe the answer is:
formidable
'redoubtable' is the definition.
(synonyms)
'for central sailor left half' is the wordplay.
'central' becomes 'mid' (I've seen this in another clue).
'sailor' becomes 'ab' (abbreviation for able seaman).
'half' indicates taking half.
'left' cut in half is 'le'.
'for'+'mid'+'ab'+'le'='FORMIDABLE'
'is' is the link.
(Other definitions for formidable that I've seen before include "Intimidatory" , "Awesome" , "Threatening, challenging" , "Intimidating" .)