Loyal Irishman in charge imprisons threesome (9)
I believe the answer is:
patriotic
'loyal' is the definition.
('patriotic' can be a synonym of 'loyal')
'irishman in charge imprisons threesome' is the wordplay.
'irishman' becomes 'pat'.
'in charge' becomes 'ic'.
'imprisons' means one lot of letters goes inside another.
'threesome' becomes 'trio' ('trio' can be a synonym of 'threesome').
'pat'+'ic'='patic'
'patic' enclosing 'trio' is 'PATRIOTIC'.
(Other definitions for patriotic that I've seen before include "Supporting the state" , "Devoted to one's country" , "flag-waving?" , "Loyal" , "Having love for one's country" .)