Start of play about Waterloo, maybe small prelude before battle (6,8)
I believe the answer is:
action stations
'prelude before battle' is the definition.
I can't judge whether this defines the answer.
'start of play about waterloo maybe small' is the wordplay.
'start of play' becomes 'act i' (Act I).
'about' becomes 'on' (as in a book 'on' a certain topic).
'waterloo maybe' becomes 'station' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'small' becomes 's' (abbreviation - e.g. clothes size).
'acti'+'on'+'station'+'s'='ACTION STATIONS'
Can you help me to learn more?
(Other definitions for action stations that I've seen before include "prepare for a battle!" , "ready for a fight!" , "Warning to be ready" , "Get in position to fight" , "Warning to take up position" .)