Batman's sidekick broke biro on first night (5)
I believe the answer is:
robin
'batman's sidekick' is the definition.
(I've seen this in another clue)
'broke biro on first night' is the wordplay.
'broke' is an anagram indicator.
'on' is a charade indicator (letters next to each other).
'first night' becomes 'n' (1st letter of 'night').
'biro' anagrammed gives 'robi'.
'robi'+'n'='ROBIN'
(Other definitions for robin that I've seen before include "Bird; Batman's aide" , "So-called outlaw" , "Batman and --" , "Batman's assistant" , "Bird - one in Lincoln green?" .)