During satire, get niece to send up this number (7)
I believe the answer is:
integer
'this number' is the definition.
(integer is a kind of number)
'during satire get niece to send up' is the wordplay.
'during' indicates a hidden word.
'to send up' says the letters should be written in reverse.
'regetni' is hidden within 'satire get niece'.
'regetni' backwards is 'INTEGER'.
(Other definitions for integer that I've seen before include "Say one" , "A whole number, not a fraction" , "Get Erin (anag)" , "Gin tree (anag)" , "one or two, perhaps" .)