Unreliable time indicator united island in revolt (7)
I believe the answer is:
sundial
'unreliable time indicator' is the definition.
'sundial' can be an answer for 'indicator' (I have seen 'Indicator of time ' mean 'sundial' so perhaps 'indicator' could also mean 'sundial'). I'm not sure about the 'unreliable time' bit.
'united island in revolt' is the wordplay.
'united' becomes 'U' (abbreviation as in Man U for Manchester United).
'in revolt' indicates anagramming the letters.
'u'+'island'='uisland'
'uisland' is an anagram of 'SUNDIAL'.
Can you help me to learn more?
(Other definitions for sundial that I've seen before include "Instrument showing the time" , "Old timer?" , "Garden timepiece" , "Ancient timepiece" , "it'll tell you the time" .)