Proportion carried by horse all the time (8)
I believe the answer is:
duration
'time' is the definition.
(duration is a kind of time)
'proportion carried by horse all' is the wordplay.
'proportion' becomes 'ratio' (I've seen this in another clue).
'carried by' means one lot of letters goes inside another.
'horse all' becomes 'dun' (I've seen this before. I am not sure about the 'all' bit.).
'ratio' going within 'dun' is 'DURATION'.
'the' is the link.
Can you help me to learn more?
(Other definitions for duration that I've seen before include "Time scale" , "Period during which something continues" , "Around it (anag.)" , "Term, span, time" , "Length of time" .)