Sides of invention fold up (8)
I believe the answer is:
increase
'up' is the definition.
(I've seen this in another clue)
'sides of invention fold' is the wordplay.
'sides of' suggests removing the centre (only the letters on the sides remain).
'fold' becomes 'crease' (creasing is a kind of folding).
'invention' with its centre removed is 'in'.
'in'+'crease'='INCREASE'
(Other definitions for increase that I've seen before include "Become greater in size" , "Grow in size" , "Grow, add to" , "Make (amount) greater" , "Breed - nicer sea (anag)" .)