Interim ruler takes time - I have to be making new growth (12)
I believe the answer is:
regenerative
'making new growth' is the definition.
I can't judge whether this defines the answer.
'interim ruler takes time i have to' is the wordplay.
'interim ruler' becomes 'regent' (a regent rules on behalf of a monarch).
'takes' is an insertion indicator.
'time' becomes 'era' (era is a kind of time).
'i have to' becomes 'ive' (I am not sure about this - if you are sure you should believe this answer much more).
'regent' placed around 'era' is 'regenerat'.
'regenerat'+'ive'='REGENERATIVE'
'be' is the link.
Can you help me to learn more?
(Another definition for regenerative that I've seen is " Promoting renewal".)