Symbol in register, a circle (4)
I believe the answer is:
logo
'symbol' is the definition.
(logo is a kind of symbol)
'register a circle' is the wordplay.
'register' becomes 'log' (I've seen this before).
'a circle' becomes 'o'.
'log'+'o'='LOGO'
'in' is the link.
(Other definitions for logo that I've seen before include "Emblematic design" , "Company emblem or device" , "Brand's emblem" , "Symbol or emblem used by organisation" , "Emblem, trade-mark" .)