Pass water day back, seeing bass (4)
I believe the answer is:
deep
'bass' is the definition.
(I know that bass can be written as deep)
'pass water day back' is the wordplay.
'pass water' becomes 'pee' (synonyms).
'day' becomes 'd' (abbreviation for day).
'back' is a reversal indicator.
'pee'+'d'='peed'
'peed' reversed gives 'DEEP'.
'seeing' acts as a link.
(Other definitions for deep that I've seen before include "profoundly" , "Wise - difficult to understand" , "(Of sound) low in pitch" , "dangerous for swimming?" , "Cavernous" .)