Black and green material from some shell suits (7)
I believe the answer is:
becomes
'suits' is the definition.
('become' can be a synonym of 'suit')
'black and green material from some shell' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'and' says to put letters next to each other.
'green' becomes 'eco' (eco-friendly).
'material from some shell' becomes 'mes' (I can't justify this - if you can you should believe this answer much more).
'b'+'eco'+'mes'='BECOMES'
Can you help me to learn more?
(Other definitions for becomes that I've seen before include "Suits, befits" , "Changes into" , "Undergoes a change" , "Turns into" , "Gets" .)