Clear obstacle in net at sea (13)
I believe the answer is:
incontestable
'clear' is the definition.
(both can mean that something can't be argued with)
'obstacle in net at sea' is the wordplay.
'at sea' indicates an anagram.
'obstacle'+'in'+'net'='obstacleinnet'
'obstacleinnet' is an anagram of 'INCONTESTABLE'.
(Other definitions for incontestable that I've seen before include "Beyond dispute" , "sure" , "Without doubt" .)