Mighty relic abandoned by priest in weakness (8)
I believe the answer is:
forcible
'mighty' is the definition.
(thesaurus)
'relic abandoned by priest in weakness' is the wordplay.
'abandoned by' is a deletion indicator.
'priest' becomes 'eli' (biblical figure).
'in' is an insertion indicator.
'weakness' becomes 'foible' (a foible is a weakness or failing).
'relic' with 'eli' taken away is 'rc'.
'rc' inserted into 'foible' is 'FORCIBLE'.
(Other definitions for forcible that I've seen before include "Vehement" , "Convincing" , "Compulsory" , "too violent?" , "Compelling" .)