Grew quickly when strike finished (4,2)
I believe the answer is:
shot up
'grew quickly' is the definition.
(I've seen this in another clue)
'strike finished' is the wordplay.
'strike' becomes 'shot' (I can't justify this - if you can you should give a lot more credence to this answer).
'finished' becomes 'up' (eg 'time's up' means the time is finished).
'shot'+'up'='SHOT UP'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for shot up that I've seen before include "very quickly got high" , "Grew quickly" .)