Present in antimony and everything but mushroom (8)
I believe the answer is:
snowball
'mushroom' is the definition.
(I've seen this before)
'present in antimony and everything' is the wordplay.
'present' becomes 'now' (both can mean the current period).
'in' indicates putting letters inside.
'antimony' becomes 'Sb' (Sb is the chemical symbol for antimony).
'and' is a charade indicator (letters next to each other).
'everything' becomes 'all' (all things).
'now' placed within 'sb' is 'snowb'.
'snowb'+'all'='SNOWBALL'
'but' is the link.
(Other definitions for snowball that I've seen before include "Throw it in winter" , "Cold missile - advocaat and lemonade cocktail" , "Cold sphere" , "Get relentlessly larger" , "Increase at a rapidly accelerating rate, in winter?" .)