Turn trademark over for large number (6)
I believe the answer is:
googol
'large number' is the definition.
(thesaurus)
'turn trademark over' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'trademark' becomes 'logo' (logo is a kind of trademark).
'over' says the letters should be written backwards.
'logo' reversed gives 'ogol'.
'go'+'ogol'='GOOGOL'
'for' is the link.
(Other definitions for googol that I've seen before include "Vast figure" , "There's a huge amount!" , "unimaginable figure" , "Ten to the power of 100" , "very large number" .)