Pair getting aboard yesterday (6)
I believe the answer is:
pronto
'yesterday' is the definition.
(I've seen this in another clue)
'pair getting aboard' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'getting' means one lot of letters go next to another.
'aboard' becomes 'on to' (I've seen this in another clue).
'pr'+'onto'='PRONTO'
(Other definitions for pronto that I've seen before include "Quickly, amigo" , "Right now, immediately" , "Right now, amigo" , "Quickly, without delay" , "Double quick!" .)