Turning over note to obtain National Insurance number (7)
I believe the answer is:
integer
'number' is the definition.
(I know that integer is a type of number)
'turning over note to obtain national insurance' is the wordplay.
'turning over' is a reversal indicator.
'note' becomes 're' (as in do-re-mi).
'to obtain' becomes 'get' ('get' can be a synonym of 'obtain').
'national insurance' becomes 'NI' (UK tax).
're'+'get'+'ni'='regetni'
'regetni' back-to-front is 'INTEGER'.
(Other definitions for integer that I've seen before include "one or two, perhaps" , "Say one" , "rational number remains" , "The whole thing" , "Perhaps 60,000" .)