Mere mortal Hera struck infertile in the end (4)
I believe the answer is:
lake
'mere' is the definition.
(I've seen this before)
'mortal hera struck infertile in the end' is the wordplay.
'in the end' suggests the final letters.
The final letters of 'mortal hera struck infertile' are 'LAKE'.
(Other definitions for lake that I've seen before include "Excess of wine?" , "Visible representation" , "Eg, Windermere" , "Water in North Italy" , "Purplish-red pigment" .)