First cardinal performing service at the end (3)
I believe the answer is:
one
'first' is the definition.
(I've seen this in another clue)
'cardinal performing service at the end' is the wordplay.
'cardinal performing' becomes 'on' (as in being on stage. I am not sure about the 'cardinal' bit.).
'at the end' suggests the final letters.
The last letter of 'service' is 'e'.
'on'+'e'='ONE'
Can you help me to learn more?
(Other definitions for one that I've seen before include "SECOND PART OF 16" , "Single, unitary" , "joke" , "1" , "If upper-class, perhaps I" .)