Crikey — jack held by boy playing against Spain (2,4)
I believe the answer is:
by jove
'crikey' is the definition.
(both are expressions of surprise)
'jack held by boy playing against spain' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'held by' is an insertion indicator.
'playing' indicates anagramming the letters.
'against' becomes 'v' (abbreviation for 'versus').
'spain' becomes 'e' (abbreviation for Espana).
'boy' is an anagram of 'byo'.
'j' put inside 'byo' is 'byjo'.
'byjo'+'v'+'e'='BY JOVE'
(Other definitions for by jove that I've seen before include "Well, I'll be blowed!" , "Good God!" , "Well, I'm blowed!" , "Good gracious!" , "Exclamation of admiration" .)