A course of action over time becomes offensive (6)
I believe the answer is:
attack
'offensive' is the definition.
('attack' can be similar in meaning to 'offensive')
'a course of action over time' is the wordplay.
'course of action' becomes 'tack' (I've seen this before).
'over' is an insertion indicator.
'time' becomes 't'.
'tack' enclosing 't' is 'ttack'.
'a'+'ttack'='ATTACK'
'becomes' is the link.
(Other definitions for attack that I've seen before include "Strike" , "Launch an offensive" , "Assault - assail" , "Offensive or inslaught" , "Act violently against or criticise strongly" .)