Held back (6,2)
There are a few possibilities for this:
reined in
'held back' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for detained that I've seen before include "Under restraint" , "Deaden it (anag.)" , "Delayed or confined" , "Held in Custody" , "Remanded or delayed" .)
'held back' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for retarded that I've seen before include "Delayed, slowed down" , "Caused delay to, slowed progress" , "Underdeveloped" .)
(Other definitions for reined in that I've seen before include "Stopped or slowed up, a horse perhaps" , "Slowed up one's horse" , "Curbed" , "'Slowed, checked (6,2)'" , "Checked - stopped" .)