Growth in understanding (12)
I believe the answer is:
appreciation
This clue is a double definition.
'growth' is the first definition.
(thesaurus)
'understanding' is the second definition.
(I know that appreciation is a type of understanding)
'in' is the link.
(Other definitions for appreciation that I've seen before include "recognition" , "An expression of gratitude" , "Gratitude; increase in value" , "Rise" , "It is shown by applause" .)