Support for a singer getting out of line (5)
I believe the answer is:
basis
'support for' is the definition.
(I've seen this before)
'a singer getting out of line' is the wordplay.
'a singer' becomes 'bass' (bass is a kind of singer).
'getting' means one lot of letters goes inside another.
'out of line' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'bass' placed around 'i' is 'BASIS'.
Can you help me to learn more?
(Other definitions for basis that I've seen before include "Reason" , "Principal element" , "grounds" , "First principle" , "Underlying support (for an idea)" .)