Collected a horse barrier in crimson (10)
I believe the answer is:
aggregated
'collected' is the definition.
'a horse barrier in crimson' is the wordplay.
'horse' becomes 'gg'.
'barrier' becomes 'gate' (gate is a kind of barrier).
'in' means one lot of letters goes inside another.
'crimson' becomes 'red' (synonyms).
'gate' inserted within 'red' is 'regated'.
'a'+'gg'+'regated'='AGGREGATED'
(Other definitions for aggregated that I've seen before include "Combined into a whole" , "Made to go to mass" .)