A large number try returning small design (6)
I believe the answer is:
googol
'a large number' is the definition.
'try returning small design' is the wordplay.
'try' becomes 'go' (go is a kind of try).
'returning' is a reversal indicator.
'small design' becomes 'logo' (I've seen this before).
'logo' written backwards gives 'ogol'.
'go'+'ogol'='GOOGOL'
(Other definitions for googol that I've seen before include "Vast figure" , "10100" , "extraordinary figure" , "There's a huge amount!" , "Big number" .)