Because it's held by a loose rein? False! (9)
I believe the answer is:
insincere
'false' is the definition.
('insincere' is associated in meaning with 'false')
'because it's held by a loose rein?' is the wordplay.
'because' becomes 'since' (I've seen this before).
'it's held by' indicates putting letters inside.
'a loose' indicates anagramming the letters (letters are loosened from their original positions).
'rein' is an anagram of 'inre'.
'since' put within 'inre' is 'INSINCERE'.
(Other definitions for insincere that I've seen before include "False, hypocritical" , "Feigned, smarmy" , "Less than honest" , "displaying hypocrisy" , "Hollow" .)