Mounts first of rings in flexible pipes (6)
I believe the answer is:
horses
'mounts' is the definition.
(I've seen this in another clue)
'first of rings in flexible pipes' is the wordplay.
'first of rings' becomes 'r' (1st letter of 'rings').
'in' is an insertion indicator.
'flexible pipes' becomes 'hoses' (flexible pipes carrying water).
'r' going inside 'hoses' is 'HORSES'.
(Other definitions for horses that I've seen before include "Equine beasts" , "Domesticated animals" , "Hoofed mammals (which go with 19 4)" , "Equines" , "See 1" .)