Straighten out the seaweed? (8)
I believe the answer is:
untangle
'straighten' is the definition.
(I've seen this in another clue)
'out the seaweed?' is the wordplay.
'out' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'the seaweed?' becomes 'tangle' (tangle is a type of seaweed).
'un'+'tangle'='UNTANGLE'
(Other definitions for untangle that I've seen before include "Sort out" , "Straighten out, free from mesh" , "'Unpick, unravel (8)'" , "Extricate" , "Make free from disorder" .)