Certain sailors appearing with old instrument (8)
I believe the answer is:
absolute
'certain' is the definition.
(I've seen this before)
'sailors appearing with old instrument' is the wordplay.
'sailors' becomes 'abs' (AB is short for able seaman).
'appearing with' says to put letters next to each other.
'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 "for certain" , "Complete and unrestricted, say power" , "Not qualified; total" , "Total, utter" , "Complete or unconditional" .)