Download tip backing team: it may be found in a book (8)
I believe the answer is:
appendix
'it may be found in a book' is the definition.
The definition suggests a singular noun which matches the answer.
'download tip backing team' is the wordplay.
'download' becomes 'app' (I can't explain this - if you can you should give a lot more credence to this answer).
'tip' becomes 'end' (tip can mean the end of something).
'backing' says the letters should be written in reverse.
'team' becomes 'XI' (a football team, from Roman numerals for aa).
'xi' written backwards gives 'ix'.
'app'+'end'+'ix'='APPENDIX'
Can you help me to learn more?
(Other definitions for appendix that I've seen before include "Reading matter" , "Reference" , "It comes last" , "Writer's addition - may be taken out in the theatre!" , "Supplementary part" .)