Seaweed found in a millimetre of alloy (7)
I believe the answer is:
amalgam
'alloy' is the definition.
(amalgam is a kind of alloy)
'seaweed found in a millimetre' is the wordplay.
'seaweed' becomes 'alga' (singular form of 'algae').
'found in' indicates putting letters inside.
'millimetre' becomes 'mm' (synonyms).
'a'+'mm'='amm'
'alga' put inside 'amm' is 'AMALGAM'.
'of' acts as a link.
(Other definitions for amalgam that I've seen before include "Mercury alloy - combination" , "Mixture of metals" , "Mixture, blend" , "Mixture - alloy of mercury" , "A soft pliable mixture" .)