Sodium is in - I must return wave (7)
I believe the answer is:
tsunami
'wave' is the definition.
(I know that Tsunami is a type of wave)
'sodium is in i must return' is the wordplay.
'sodium' becomes 'NA' (chemical symbol).
'is in' indicates putting letters inside.
'return' shows that the letters should be reversed in order.
'i'+'must'='imust'
'imust' back-to-front is 'tsumi'.
'na' put into 'tsumi' is 'TSUNAMI'.
(Other definitions for tsunami that I've seen before include "Marine natural disaster" , "High sea wave" , "Huge wave caused by earth tremor" , "stormy sea" , "Large sea wave caused by an earthquake" .)