Store up trouble for a sea god (7)
I believe the answer is:
proteus
'a sea god' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'store up trouble' is the wordplay.
'trouble' indicates anagramming the letters.
'store'+'up'='storeup'
'storeup' anagrammed gives 'PROTEUS'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for proteus that I've seen before include "Greek sea-god" , "Sea god who assumed different shapes to avoid answering questions" .)