Calculating form of geraniums (9)
I believe the answer is:
measuring
'calculating' is the definition.
The answer and definition can be both related to thinking as well as being present participle verbs.
Perhaps you can see a link between them that I can't see?
'form of geraniums' is the wordplay.
'form of' indicates anagramming the letters (another form of the same letters).
'geraniums' is an anagram of 'MEASURING'.
Can you help me to learn more?
(Other definitions for measuring that I've seen before include "Ascertaining size of" , "Ascertaining length, breadth, etc." , "Quantifying" , "Determining the dimensions" .)