Period of fasting: hour covers good duration (6)
I believe the answer is:
length
'duration' is the definition.
(synonyms)
'period of fasting hour covers good' is the wordplay.
'period of fasting' becomes 'Lent' (fast leading up to Easter).
'hour' becomes 'h' (abbreviation).
'covers' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'lent'+'h'='lenth'
'lenth' enclosing 'g' is 'LENGTH'.
(Other definitions for length that I've seen before include "End-to-end dimension" , "Extent - distance" , "Linear extent" , "Distance from one end of an object to the other" , "Extensiveness - measure" .)