Competent sailor, leg no good? (4)
I believe the answer is:
able
'competent' is the definition.
(similar in meaning)
'sailor leg no good?' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'no' indicates named letters should be taken away.
'good?' becomes 'g' (abbreviation).
'leg' with 'g' taken away is 'le'.
'ab'+'le'='ABLE'
(Other definitions for able that I've seen before include "Capable seaman" , "displaying skill" , "- -bodied; - seaman" , "Capable or proficient" , "In a position or competent to" .)