Increase in size (6)
'increase in size' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grow that I've seen before include "Gain in size" , "Become bigger" , "Mature" , "Get bigger" , "Multiply" .)
(Other definitions for growth that I've seen before include "Process of enlargement" , "Gradual increase" , "Process of development" , "Increase - development - tumour" , "Development. maturation" .)