Pumping iron into great cast (13)
I believe the answer is:
interrogation
'pumping' is the definition.
(thesaurus)
'iron into great cast' is the wordplay.
'cast' indicates anagramming the letters.
'iron'+'into'+'great'='ironintogreat'
'ironintogreat' with letters rearranged gives 'INTERROGATION'.
(Other definitions for interrogation that I've seen before include "Formal questioning" , "Grilling" , "Close questioning" , "The third degree" , "Cross-examination" .)