Instruction to reader enthralling Reagan, possibly, at once? (6)
I believe the answer is:
pronto
'at once?' is the definition.
(pronto can mean at once or quickly)
'instruction to reader enthralling reagan' is the wordplay.
'instruction to reader' becomes 'PTO' (abbreviation for 'please turn over').
'enthralling' indicates putting letters inside.
'reagan' becomes 'ron' (I've seen this in another clue).
'pto' going around 'ron' is 'PRONTO'.
'possibly' is the link.
(Other definitions for pronto that I've seen before include "Italian telephonic greeting" , "Quickly, promptly" , "Right away, quickly" , "Right now, amigo" , "Promptly, without delay" .)