One capped once, career’s beginning to run into the sand (5)
I believe the answer is:
dunce
'sand' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'one capped once career's beginning to run into' is the wordplay.
'one capped once' becomes 'dune' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'beginning to' suggests taking the first letters.
'run into' is an insertion indicator.
The first letter of 'careers' is 'c'.
'dune' enclosing 'c' is 'DUNCE'.
'the' is the link.
Can you help me to learn more?
(Other definitions for dunce that I've seen before include "Halfwit" , "Simpleton" , "Old word for one slow at learning" , "Dull chap" , "Term of ridicule for John Dun Scotus followers" .)