Able seaman's leader knocked off (7)
I believe the answer is:
skilled
'able' is the definition.
(I've seen this before)
'seaman's leader knocked off' is the wordplay.
'leader' suggests taking the first letters.
'knocked off' becomes 'killed' (to knock off can mean to assassinate).
The first letter of 'seamans' is 's'.
's'+'killed'='SKILLED'
(Other definitions for skilled that I've seen before include "Proficient" , "Talented" , "Showing expertise" , "Able" , "Showing great aptitude" .)