Brown turning on old military alliance (4)
I believe the answer is:
nato
'military alliance' is the definition.
(I've seen this before)
'brown turning on old' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'turning' is a reversal indicator.
'on' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tan' reversed gives 'nat'.
'nat'+'o'='NATO'
(Other definitions for nato that I've seen before include "N. Atlantic alliance" , "Security agreement" , "Military/political alliance" , "Collective security alliance engaged in Bosnia" , "Military alliance (acronym)" .)