Crustacean eats last bits of seaweed before sunrise (7)
I believe the answer is:
predawn
'before sunrise' is the definition.
'crustacean eats last bits of' is the wordplay.
'crustacean' becomes 'prawn' (prawn is a kind of crustacean).
'eats' means one lot of letters goes inside another.
'last bits of' becomes 'ed' (I can't explain this - if you can you should believe this answer much more).
'prawn' going around 'ed' is 'PREDAWN'.
'seaweed' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for predawn that I've seen before include "when the night is very far spent" , "Before the sun begins to rise" , "Before first light" .)