Pale, in pain, showed sign of injury externally (8)
I believe the answer is:
bleached
'pain showed sign of injury externally' is the definition.
I can't tell whether this defines the answer.
'pale in' is the wordplay.
'pale' becomes 'bleach' (I've seen this before).
'in' becomes 'ed' (I can't explain this - if you can you should give a lot more credence to this answer).
'bleach'+'ed'='BLEACHED'
Can you help me to learn more?
(Other definitions for bleached that I've seen before include "Whitened or removed colour from" , "Paled" , "Chemically whitened" .)