After a time, go on a new course - assault (6)
I believe the answer is:
attack
'assault' is the definition.
(synonyms)
'after a time go on a new course' is the wordplay.
'after' means one lot of letters go next to another.
'a time' becomes 't' (abbreviation).
'go on' is a charade indicator (letters next to each other).
'new' indicates an anagram.
'course' becomes 'tack' (I've seen this before).
't'+'a'='ta'
'ta' is an anagram of 'at'.
'at' next to 'tack' is 'ATTACK'.
(Other definitions for attack that I've seen before include "Brownish pigment" , "Raid" , "Forward line" , "Start a fight or criticise strongly" , "Offensive or inslaught" .)