Consummate sailor's old stringed instrument (8)
I believe the answer is:
absolute
'consummate' is the definition.
(thesaurus)
'sailor's old stringed instrument' is the wordplay.
'sailor's' becomes 'ab's' (abbreviation for able seaman).
'old' becomes 'o' (abbreviation).
'stringed instrument' becomes 'lute' (lute is a type of stringed instrument).
'abs'+'o'+'lute'='ABSOLUTE'
(Other definitions for absolute that I've seen before include "Not qualified or diminished in any way" , "Complete or not limited" , "Not qualified; total" , "for certain" , "No strings attached" .)