One hiding on board tug, one overwhelmed by wave (8)
I believe the answer is:
stowaway
'one hiding on board' is the definition.
(I've seen this before)
'tug one overwhelmed by wave' is the wordplay.
'tug' becomes 'tow' (I've seen this before).
'one' becomes 'a' (a thing is one thing).
'overwhelmed by' is an insertion indicator (inserted letters get absorbed or overwhelmed).
'wave' becomes 'sway'.
'tow'+'a'='towa'
'towa' placed inside 'sway' is 'STOWAWAY'.
(Other definitions for stowaway that I've seen before include "Travel secretly" , "pack" , "Person who hides aboard a ship to get free passage" , "Person who hides on ship to avoid paying fare" , "One hiding on ship" .)