Wearing away a god, Zeus's lover, with determination ultimately (7)
I believe the answer is:
erosion
'wearing away' is the definition.
(synonyms)
'a god zeus's lover with determination ultimately' is the wordplay.
'a god' becomes 'eros' (Eros is an example).
'zeus's lover' becomes 'Io' (Io was a lover of Zeus in Greek myth).
'with' says to put letters next to each other.
'ultimately' indicates one should take the final letters.
The final letter of 'determination' is 'n'.
'eros'+'io'+'n'='EROSION'
(Other definitions for erosion that I've seen before include "being eaten away" , "dying away" , "Process of wear" , "Gradual decline" , "Wearing away (of soil, rocks etc)" .)