Large waves swamping tips of pontoon anchor (7)
I believe the answer is:
grapnel
'anchor' is the definition.
(I know that grapnel is a type of anchor)
'large waves swamping tips of pontoon' is the wordplay.
'waves' indicates anagramming the letters.
'swamping' means one lot of letters goes inside another (swamp can mean to surround or smother).
'tips of' says to hollow out the word (remove centre letters).
'pontoon' with its middle taken out is 'pn'.
'large' is an anagram of 'grael'.
'grael' placed around 'pn' is 'GRAPNEL'.
(Other definitions for grapnel that I've seen before include "Light anchor for small boats" , "Small anchor with several flukes" , "Tool with hooks for grasping and holding" , "Grappling hook" , "Hook on rope" .)