Become more active after plonk is brought round by head office (3,2)
I believe the answer is:
hot up
'become' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps you can see a link between them that I don't see?
'more active after plonk is brought round by head office' is the wordplay.
'more active after plonk' becomes 'put' (I can't justify this - if you can you should give a lot more credence to this answer).
'is brought round' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'head office' becomes 'HO'.
'put' reversed gives 'tup'.
'tup' put after 'ho' is 'HOT UP'.
Can you help me to learn more?
(Other definitions for hot up that I've seen before include "become more lively" , "more active" , "intensify" , "Become more exciting" .)