Register round trademark (4)
I believe the answer is:
logo
'trademark' is the definition.
(logo is a kind of trademark)
'register round' is the wordplay.
'register' becomes 'log' (I've seen this before).
'round' becomes 'o' (round can mean a round object).
'log'+'o'='LOGO'
(Other definitions for logo that I've seen before include "(Company) symbol" , "Symbolic design" , "Company emblem or device" , "organisational symbol" , "Corporate emblem" .)