Tsunami we validate mostly at sea (5,4)
I believe the answer is:
tidal wave
'tsunami' is the definition.
(synonyms)
'we validate mostly at sea' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'at sea' is an anagram indicator.
'we'+'validate'='wevalidate'
'wevalidate' with its final letter taken away is 'wevalidat'.
'wevalidat' with letters rearranged gives 'TIDAL-WAVE'.
(Other definitions for tidal wave that I've seen before include "Huge sea surge" , "Overwhelming surge" , "natural disaster?" , "Large movement (of water or public opinion)" , "Tsunami" .)