Before temperature fell, sun's unnatural (8)
I believe the answer is:
strained
'unnatural' is the definition.
(strained can mean false or unnatural)
'before temperature fell sun's' is the wordplay.
'before' means one lot of letters go next to another.
'temperature' becomes 't' (abbreviation).
'fell' becomes 'rained' (raining is a kind of falling).
'sun' becomes 'S' (abbreviation).
't'+'rained'='trained'
'trained' after 's' is 'STRAINED'.
(Other definitions for strained that I've seen before include "Under pressure" , "Stretched, extended to the limit" , "pushed to the limit?" , "Stretched or forced to the limit" , "taxed" .)