Negative quantity in heavenly body one mile up (5)
I believe the answer is:
minus
'negative' is the definition.
('minus' can be similar in meaning to 'negative')
'quantity in heavenly body one mile up' is the wordplay.
'quantity' becomes 'i' (I is an example**).
'in' means one lot of letters goes inside another.
'heavenly body' becomes 'sun' (Sun is an example).
'one mile' becomes 'm' (common abbreviation).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'sun'+'m'='sunm'
'i' placed into 'sunm' is 'sunim'.
'sunim' back-to-front is 'MINUS'.
(Other definitions for minus that I've seen before include "Negative sign - negative thing" , "Reduced by the subtraction of - thus?" , "Lacking; below zero" , "Less, without" , "(Of temperature) below zero" .)