Discovered deer in trouble as mount went lame (9)
I believe the answer is:
foundered
'mount went lame' is the definition.
I can't judge whether this definition defines the answer.
'discovered deer in trouble' is the wordplay.
'discovered' becomes 'found' (to discover is to find).
'in trouble' indicates anagramming the letters.
'deer' with letters rearranged gives 'ered'.
'found'+'ered'='FOUNDERED'
'as' is the link.
Can you help me to learn more?
(Another definition for foundered that I've seen is " Sank, went down".)