Seaweed penetrates a small distance as filler (7)
I believe the answer is:
amalgam
'filler' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe there's a link between them I don't understand?
'seaweed penetrates a small distance' is the wordplay.
'seaweed' becomes 'alga' (singular form of 'algae').
'penetrates' means one lot of letters goes inside another.
'small distance' becomes 'mm' (abbreviation for millimetre).
'a'+'mm'='amm'
'alga' placed within 'amm' is 'AMALGAM'.
'as' is the link.
Can you help me to learn more?
(Other definitions for amalgam that I've seen before include "mercurial combination" , "mix" , "Mixture/blend" , "Blend or combination" , "Compound" .)