Queen holds reversible one metre ruler (4)
I believe the answer is:
emir
'ruler' is the definition.
(historical term for a Muslim ruler)
'queen holds reversible one metre' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'holds' indicates putting letters inside.
'reversible' is a reversal indicator.
'one' becomes 'i' (Roman numeral).
'metre' becomes 'm' (synonyms).
'i'+'m'='im'
'im' reversed gives 'mi'.
'er' placed around 'mi' is 'EMIR'.
(Other definitions for emir that I've seen before include "Title of various rulers; I'm ER (anag.)" , "Ruler in the Middle East and mire" , "Arab leader" , "Arabic ruler" , "Local chief" .)