Able seaman's beginning attacks (4)
I believe the answer is:
fits
'attacks' is the definition.
(fit is a kind of attack)
'able seaman's beginning' is the wordplay.
'able' becomes 'fit' ('fit' can be similar in meaning to 'able'**).
'beginning' says to take the initial letters.
The initial letter of 'seamans' is 's'.
'fit'+'s'='FITS'
(Other definitions for fits that I've seen before include "Paroxysms" , "Sudden uncontrollable attacks" , "Seizures" , "Prepares" , "feels comfortable" .)