Nothing round trap for bird (6)
I believe the answer is:
linnet
'bird' is the definition.
(linnet is a kind of bird)
'nothing round trap' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'round' shows that the letters should be reversed in order.
'trap' becomes 'net' (net is a type of trap).
'nil' in reverse letter order is 'lin'.
'lin'+'net'='LINNET'
'for' acts as a link.
(Other definitions for linnet that I've seen before include "A little bird" , "Passerine bird" , "Finch so named because it feeds on flax" , "North London terminus" , "Finch formerly popular as a pet" .)