From memory, boy's gutted, given red card (2,5)
I believe the answer is:
by heart
'from memory' is the definition.
(I've seen this in another clue)
'boy's gutted given red card' is the wordplay.
'gutted' says to hollow out the word (remove centre letters) (to gut something is to remove its insides).
'given' says to put letters next to each other.
'red card' becomes 'heart' (I've seen this in another clue).
'boy' with its middle removed is 'by'.
'by'+'heart'='BY HEART'
(Other definitions for by heart that I've seen before include "To know - - is to have in the memory" , "Thoroughly memorized, word perfect" , "(Learnt) in memory" , "Committed to memory" , "Off pat" .)