Coming last is not applicable in the case of a young horse (7)
I believe the answer is:
finally
'coming last' is the definition.
'finally' can be an answer for 'last' (thesaurus). I'm not certain of the 'coming' bit.
'not applicable in the case of a young horse' is the wordplay.
'not applicable' becomes 'inall' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in the' means one lot of letters goes inside another.
'case of' suggests removing the centre (outsides of).
'a young horse' becomes 'filly' (I've seen this in another clue).
'filly' with its middle taken out is 'fy'.
'inall' put into 'fy' is 'FINALLY'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for finally that I've seen before include "when all is said and done" , "At last" , "for ever" .)