Develop; become larger (4)
I believe the answer is:
grow
I believe this clue is a double definition.
'develop' is the first definition.
(I know that develop can be written as grow)
'become larger' is the second definition.
(synonyms)
(Other definitions for grow that I've seen before include "Mature" , "Gain in size" , "Develop; increase" , "Multiply" , "Become larger" .)