Sailor brewing tea before head of steam dies down (6)
I believe the answer is:
abates
'dies down' is the definition.
(I've seen this before)
'sailor brewing tea before head of steam' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'brewing' indicates an anagram.
'before' means one lot of letters go next to another.
'head of' indicates taking the first letters.
The initial letter of 'steam' is 's'.
'tea' anagrammed gives 'ate'.
'ab'+'ate'+'s'='ABATES'
(Other definitions for abates that I've seen before include "Becomes less intense" , "lowers" , "Makes less intense" , "Becomes less strong like storm" , "Goes down like storm" .)