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 they are linked in a way I don't understand?
'form of geraniums' is the wordplay.
'form of' indicates an anagram (another form of the same letters).
'geraniums' with letters rearranged gives '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" .)