Little tree giving up nothing? One silver nutmeg, at first (5)
I believe the answer is:
ngaio
'little tree' is the definition.
(small tree of New Zealand)
'up nothing? one silver nutmeg at first' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'nothing?' becomes 'o' (looks like zero - 0).
'one' becomes 'i' (roman numeral).
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'at first' says to take the initial letters.
The first letter of 'nutmeg' is 'n'.
'o'+'i'+'ag'+'n'='oiagn'
'oiagn' back-to-front is 'NGAIO'.
'giving' acts as a link.
(Other definitions for ngaio that I've seen before include "foreign tree" , "New Zealand tree" , "wood" , "NZ tree; in Goa (anag.)" , "- Marsh, NZ author" .)