A three originally on the Spanish flag (4,5)
I believe the answer is:
lose heart
'flag' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'a three originally on the spanish' is the wordplay.
'originally' indicates an anagram.
'on' indicates putting letters inside (as in clothing 'on' a person).
'the spanish' becomes 'los' ('the' in Spanish).
'a'+'three'='athree'
'athree' anagrammed gives 'eheart'.
'eheart' placed around 'los' is 'LOSE HEART'.
Can you help me to learn more?
(Other definitions for lose heart that I've seen before include "Mislay a card" , "Falter in courage - or fall in love? (4, 5)" , "Start to despair" , "Become despondent" , "Let hopes dwindle - real ethos involved" .)