Like Martin, he's flighty (5)
I believe the answer is:
robin
'like martin' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'he's flighty' is the wordplay.
'he' becomes 'Rob'.
'flighty' becomes 'in' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rob'+'in'='ROBIN'
Can you help me to learn more?
(Other definitions for robin that I've seen before include "Friendly winter bird" , "See 12" , "Hood, Goodfellow or redbreast?" , "Traditional Christmas card bird" , "Red-breasted Christmas card bird" .)