Seaweed beginning to trap fish (6)
I believe the answer is:
tangle
'seaweed' is the definition.
(tangle is a type of seaweed)
'beginning to trap fish' is the wordplay.
'beginning to' suggests taking the first letters.
'fish' becomes 'angle' (angling is a kind of fishing).
The initial letter of 'trap' is 't'.
't'+'angle'='TANGLE'
(Other definitions for tangle that I've seen before include "Knotted, twisted mass" , "Complicated mess" , "Jumble" , "Twist into a confused mass, or informally fight with" , "Entwine, snarl" .)