One anchored at sea sparks up joint (9)
I believe the answer is:
lightship
'one anchored at sea' is the definition.
(type of ship)
'sparks up joint' is the wordplay.
'sparks up' becomes 'lights'.
'joint' becomes 'hip' (hip joint is a kind of joint).
'lights'+'hip'='LIGHTSHIP'
(Other definitions for lightship that I've seen before include "Moored vessel warning sailors of a hazard" , "The guiding vessel appears not to have much weight" , "Navigational aid" , "vessel anchored near rocks?" , "could reveal a watery problem" .)