A gradual increase in addition (2,4)
I believe the answer is:
as well
'in addition' is the definition.
(I've seen this before)
'a gradual increase' is the wordplay.
'gradual increase' becomes 'swell' (swelling is a kind of increasing. I am not sure about the 'gradual' bit.).
'a'+'swell'='AS WELL'
Can you help me to learn more?
(Other definitions for as well that I've seen before include "A swell sort of addition" , "Appropriate" , "with equal effect" , "Also, in addition" , "on top" .)