Subject to rising and falling temperature, one boy suffering setback (5)
I believe the answer is:
tidal
'subject to rising and falling' is the definition.
(as in the tides)
'temperature one boy suffering setback' is the wordplay.
'temperature' becomes 't' (abbreviation).
'one' becomes 'i' (Roman numeral).
'boy' becomes 'lad' (informal term for boy).
'suffering setback' shows that the letters should be reversed in order.
'lad' reversed gives 'dal'.
't'+'i'+'dal'='TIDAL'
(Other definitions for tidal that I've seen before include "Like waves" , "Subject to moon's pull" , "To do with movement of seas" , "Like the sea" , "Concerning the rise and fall of the sea" .)