One third of trimester with head away, I get busy (2,3,2)
I believe the answer is:
on the go
'busy' is the definition.
(similar in meaning)
'one third of trimester with head away i' is the wordplay.
'one third of trimester' becomes 'month' (a trimester is a period of three months).
'with head away' means to remove the first letter.
'i' becomes 'ego' (I've seen this in other clues).
'month' with its first letter removed is 'onth'.
'onth'+'ego'='ON THE GO'
'get' acts as a link.
(Other definitions for on the go that I've seen before include "Very active (colloq.)" , "Busily active" , "Active and energetic" , "Moving -- active" , "ready to start monopoly?" .)