Singer is allowed to alter opening lines (6)
I believe the answer is:
canary
'singer' is the definition.
(I know that canary is a type of singer)
'is allowed to alter opening lines' is the wordplay.
'is allowed to' becomes 'can' (if you're allowed to do something, you can do it).
'opening' suggests taking the first letters.
'lines' becomes 'ry' (abbreviation for railway).
The initial letter of 'alter' is 'a'.
'can'+'a'+'ry'='CANARY'
(Other definitions for canary that I've seen before include "Type of finch" , "Yellow singing bird" , "Popular cage bird, often yellow" , "Bright yellow shade" , "a sort of finch" .)