Retainer tries to conceal being inactive (5)
I believe the answer is:
inert
'inactive' is the definition.
(similar in meaning)
'retainer tries to conceal' is the wordplay.
'to conceal' indicates a hidden word.
'INERT' is hidden in the letters of 'retainer tries'.
'being' is the link.
(Other definitions for inert that I've seen before include "Like some gases" , "Devoid of motion or change" , "like nitrogen" , "Sluggish, inactive" , "Passive, immobile" .)