Claim skill keeps ship to course, ultimately (6)
I believe the answer is:
assert
'claim' is the definition.
(thesaurus)
'skill keeps ship to course ultimately' is the wordplay.
'skill' becomes 'art' (I've seen this before).
'keeps' is an insertion indicator.
'ship' becomes 'ss'.
'to' says to put letters next to each other.
'ultimately' suggests the final letters.
The final letter of 'course' is 'e'.
'ss'+'e'='sse'
'art' placed around 'sse' is 'ASSERT'.
(Other definitions for assert that I've seen before include "Confidently say" , "Swear" , "Contend, maintain" , "Express forcibly" , "Declare, profess" .)