Fungus by the sea gets wet, finally (5)
I believe the answer is:
yeast
'fungus' is the definition.
(I know that yeast is a type of fungus)
'by the sea gets wet finally' is the wordplay.
'finally' says to take the final letters.
The last letters of 'by the sea gets wet' are 'YEAST'.
(Other definitions for yeast that I've seen before include "Baker's ingredient" , "Brewing agent" , "fermenter" , "What makes bread rise" , "Dough raising agent" .)