Period of abstinence for firm (4)
I believe the answer is:
fast
I believe this is a double definition.
'period of abstinence' is the first definition.
(abstaining from food)
'firm' is the second definition.
(I know that firm can be written as fast)
'for' is the link.
(Other definitions for fast that I've seen before include "Exercise for spiritual wellbeing" , "Speedily - secured" , "Unlikely to run" , "requiring only brief exposure" , "Period without food" .)