Outlets welcoming writing by nags (6)
I believe the answer is:
horses
'nags' is the definition.
'outlets welcoming writing' is the wordplay.
'outlets' becomes 'hoses'.
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'writing' becomes 'r' (I've seen this in other clues).
'hoses' enclosing 'r' is 'HORSES'.
'by' is the link.
(Other definitions for horses that I've seen before include "Hoofed mammals (which go with 19 4)" , "Domesticated animals" , "Steeds" , "''If wishes were . . . . . ., beggars would ride''" , "Equine beasts" .)