Successor holding iron cow? (6)
I believe the answer is:
heifer
'cow?' is the definition.
(heifer is a kind of cow)
'successor holding iron' is the wordplay.
'successor' becomes 'heir' (synonyms).
'holding' indicates putting letters inside.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'heir' placed around 'fe' is 'HEIFER'.
(Other definitions for heifer that I've seen before include "Female bovine that has not given birth" , "Young cow which has not yet given birth" , "Farm animal" , "beast" , "a little lower" .)