Middle-distance specialist once ran, briefly, in 100m (4)
I believe the answer is:
cram
'middle-distance specialist once' is the definition.
I can't judge whether this definition defines the answer.
'ran briefly in 100m' is the wordplay.
'briefly' means to remove the last letter.
'in' means one lot of letters goes inside another.
'c00m' becomes 'cm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ran' with its final letter taken away is 'ra'.
'ra' inserted into 'cm' is 'CRAM'.
Can you help me to learn more?
(Other definitions for cram that I've seen before include "Pack in or study hard" , "jam" , "Feed greedily" , "Press together" , "A crush" .)