They're in the sky some time rising over British isle (5)
I believe the answer is:
nimbi
'they're in the sky' is the definition.
(plural of nimbus, type of cloud)
'some time rising over british isle' is the wordplay.
'some time' becomes 'min' (short for minute).
'rising' says the letters should be written backwards (going up in a down clue).
'over' means one lot of letters go next to another.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'isle' becomes 'i'.
'min' in reverse letter order is 'nim'.
'nim'+'b'+'i'='NIMBI'
(Other definitions for nimbi that I've seen before include "Rain-clouds" , "Dark things" , "They may surround the head" , "Rain-bearing clouds" , "rings" .)