Take part in a seaside activity and risk a wet undoing? (5-3)
I believe the answer is:
water-ski
'take part in a seaside activity' is the definition.
'risk a wet undoing?' is the wordplay.
'undoing?' indicates an anagram.
'risk'+'a'+'wet'='riskawet'
'riskawet' with letters rearranged gives 'WATER-SKI'.
'and' acts as a link.
(Other definitions for water-ski that I've seen before include "Get towed behind a motorboat" , "Be towed behind a speedboat" , "To ride behind a speedboat" , "Item used in aquatic sport" , "Tweak sir (anag)" .)