Inertia allowed Henry and Gary out (8)
I believe the answer is:
lethargy
'inertia' is the definition.
(I know that lethargy is a type of inertia)
'allowed henry and gary out' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'and' means one lot of letters go next to another.
'out' indicates an anagram (out can mean wrong or inaccurate).
'gary' is an anagram of 'argy'.
'let'+'h'+'argy'='LETHARGY'
(Other definitions for lethargy that I've seen before include "Lack of vitality or energy" , "Lassitude, languor" , "Drowsy" , "Inertia" , "'Lassitude, langour (8)'" .)