Possible Clues:

• One of the two projections close to a cannon’s centre of gravity
• Turn in, off and on, part of a cannon
• Moving in, turn on one of a pair of pivots
• One of pair of supports on which something pivots
• Pin in, turn switches on
• Supporting pin
• After tea, manage Ulster on with the support of this
• Any number in workers' organisation giving treasurer support - or part of it?
• Turn in on (anag)