King in glasses waves (8)
I believe the answer is:
breakers
'waves' is the definition.
(breaker is a kind of wave)
'king in glasses' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'in' means one lot of letters goes inside another.
'glasses' becomes 'beakers' (beaker is a type of glass vessel).
'r' placed inside 'beakers' is 'BREAKERS'.
(Other definitions for breakers that I've seen before include "Large waves or wreckers" , "White-topped waves" , "demolition men" , "Large waves beloved of surfers" , "Smashing types" .)