Cromwell, for example, shot a revolutionary when holding mass (6)
I believe the answer is:
thomas
'cromwell for example' is the definition.
(Thomas Cromwell, Tudor statesman)
'shot a revolutionary when holding mass' is the wordplay.
'revolutionary' indicates an anagram.
'when holding' indicates putting letters inside.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'shot'+'a'='shota'
'shota' anagrammed gives 'thoas'.
'thoas' going around 'm' is 'THOMAS'.
(Other definitions for thomas that I've seen before include "The Blue Boy painter, d. 1788" , "Rev. Awdry's Tank Engine" , "fellow" , "The Apostle who initially doubted the Resurrection" , "Famous tank engine" .)