Certain sailor with stringed instrument below in case (8)
I believe the answer is:
absolute
'certain' is the definition.
(I've seen this before)
'sailor with stringed instrument below in case' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'with' says to put letters next to each other.
'stringed instrument' becomes 'lute' (lute is a kind of stringed instrument).
'below' is a charade indicator (letters next to each other).
'in case' becomes 'so'.
'lute' after 'so' is 'solute'.
'ab' put next to 'solute' is 'ABSOLUTE'.
(Other definitions for absolute that I've seen before include "Sheer, total, not limited" , "Complete or not limited" , "Rival" , "Peremptory" , "Free from limits, power perhaps" .)