Dream I'm in boat at sea (8)
I believe the answer is:
ambition
'dream' is the definition.
(synonyms)
'i'm in boat at sea' is the wordplay.
'at sea' is an anagram indicator.
'im'+'in'+'boat'='iminboat'
'iminboat' is an anagram of 'AMBITION'.
(Other definitions for ambition that I've seen before include "Target" , "wanting to succeed" , "Strong desire to achieve, fame or fortune" , "Cherished aspiration" , "Drive to succeed" .)