Certain sailor's old instrument (8)
I believe the answer is:
absolute
'certain' is the definition.
(I've seen this before)
'sailor's old instrument' is the wordplay.
'sailor's' becomes 'abs' (abbreviation for able seaman).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'instrument' becomes 'lute' (lute is a kind of instrument).
'abs'+'o'+'lute'='ABSOLUTE'
(Other definitions for absolute that I've seen before include "unrestricted by constitutional checks" , "Unlimited; not relative" , "Peremptory" , "Complete or not limited" , "Unconditional, downright," .)