Suppress cool hydrogen, one small amount (7)
I believe the answer is:
inhibit
'suppress' is the definition.
(I know that suppress can be written as inhibit)
'cool hydrogen one small amount' is the wordplay.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'one' becomes 'i' (Roman numeral).
'small amount' becomes 'bit' (I've seen this in another clue).
'in'+'h'+'i'+'bit'='INHIBIT'
(Other definitions for inhibit that I've seen before include "Curb" , "Limit the range or extent of" , "Repress" , "Hold back" , "Prevent expression of" .)