Unreliable traitor held by top lady in charge (7)
I believe the answer is:
erratic
'unreliable' is the definition.
(similar in meaning)
'traitor held by top lady in charge' is the wordplay.
'traitor' becomes 'rat' (I've seen this in another clue).
'held by' is an insertion indicator.
'top lady' becomes 'er' (Elizabeth Regina, Queen Elizabeth).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'er'+'ic'='eric'
'rat' inserted into 'eric' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Unstable" , "Unpredictable - car tire (anag)" , "Inconstant" , "Unpredictable, temperamental" , "Irregular, capricious" .)