Front of boat lifted, belly occasionally tremulous (6)
I believe the answer is:
wobbly
'tremulous' is the definition.
(thesaurus)
'front of boat lifted belly occasionally' is the wordplay.
'front of boat' becomes 'bow' (nautical term).
'lifted' is a reversal indicator (going up in down clue).
'occasionally' means one should take alternating letters.
The alternate letters of 'belly' are 'bly'.
'bow' written backwards gives 'wob'.
'wob'+'bly'='WOBBLY'
(Other definitions for wobbly that I've seen before include "one may be thrown in temper" , "Rickety" , "Swaying unsteadily" , "Unstable - tantrum" , "Unstable - it may be thrown!" .)