Bird beginning to roost on old rubbish receptacle (5)
I believe the answer is:
Here is my best explanation:
'bird' is the definition.
(robin is a kind of bird)
'beginning to roost on old rubbish receptacle' is the wordplay.
'beginning to' says to take the initial letters.
'on' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rubbish receptacle' becomes 'bin' (I've seen this before).
The initial letter of 'roost' is 'r'.
(Other definitions for robin that I've seen before include "Bird; man's name", "Redbreast", "A small bird", "Bird - boy's name", "Small songbird", "flier", "Hood, possibly", "Christmas songbird", "Red and brown songbird" and "Friendly winter bird".)