Special sailor's regret? To retire destroyer (8)
I believe the answer is:
saboteur
'destroyer' is the definition.
(a saboteur destroys something)
'special sailor's regret? to retire' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'sailor' becomes 'ab' (abbreviation for able seaman).
'regret?' becomes 'rue' (synonyms).
'retire' says the letters should be written in reverse.
'rue'+'to'='rueto'
'rueto' written backwards gives 'oteur'.
's'+'ab'+'oteur'='SABOTEUR'
(Other definitions for saboteur that I've seen before include "One who deliberately subverts" , "Vandal" , "Our beats (anag) - fifth columnist" , "One who wrecks" , "Abuser to become a kind of destroyer" .)