Prepared to act -- that must change before day breaks (2,3,5)
I believe the answer is:
at the ready
'prepared to act that must change before day breaks' is the definition.
'at the ready' can be an answer for 'prepared' (I've seen this before). I am not certain of the rest of the definition.
'that must change before day breaks' is the wordplay.
'must change' indicates an anagram (I've seen this in other clues).
'before' becomes 'ere' (poetic term).
'breaks' indicates anagramming the letters.
'that' anagrammed gives 'atth'.
'day' is an anagram of 'ady'.
'atth'+'ere'+'ady'='AT THE READY'
Can you help me to learn more?
(Other definitions for at the ready that I've seen before include "Poised for action" , "Available for use in operating" , "on standby" , "Prepared for instant action" , "Prepared or available for immediate use" .)