Wiktionary
whipper-in
n. 1 A huntsman who keeps the hounds from wandering, and whips them in, if necessary, to the chase. 2 (context politics English) One who enforces the discipline of a party, and urges the attendance and support of the members on all necessary occasions.
WordNet
whipper-in
n. huntsman's assistant in managing the hounds
[also: whippers-in (pl)]