Keen in the morning to stop period of fasting (6)
I believe the answer is:
lament
'keen' is the definition.
(keen can mean to wail with grief)
'morning to stop period of fasting' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'to stop' indicates putting letters inside (stopping can mean blocking a gap).
'period of fasting' becomes 'Lent' (fast leading up to Easter).
'am' put inside 'lent' is 'LAMENT'.
'in the' is the link.
(Other definitions for lament that I've seen before include "Mourn, express regret" , "Express disappointment" , "Sad song" , "Song expressing grief" , "Express sorrow" .)