Blush when disheartened girl expresses pain (4)
I believe the answer is:
glow
'blush' is the definition.
(I've seen this in another clue)
'disheartened girl expresses pain' is the wordplay.
'disheartened' suggests removing the centre (the middle or heart is removed).
'expresses pain' becomes 'ow' (I can't justify this - if you can you should give a lot more credence to this answer).
'girl' with its middle removed is 'gl'.
'gl'+'ow'='GLOW'
'when' is the link.
Can you help me to learn more?
(Other definitions for glow that I've seen before include "Emit light and heat without flame" , "Shine with warmth, emotion" , "Radiate or redden" , "Give out steady light" , "rosy colour" .)