Exciting hour in allotment work (9)
I believe the answer is:
thrilling
'exciting' is the definition.
(thrilling is a kind of exciting)
'hour in allotment work' is the wordplay.
'hour' becomes 'hr' (abbreviation).
'in' is an insertion indicator.
'allotment work' becomes 'tilling' (tilling might be done on an allotment).
'hr' placed inside 'tilling' is 'THRILLING'.
(Other definitions for thrilling that I've seen before include "Causing pleasurable excitement" , "Hair-raising" , "Sensational" , "Engrossing" , "Extremely exciting" .)