At church, Nancy's part is uncertain (6)
I believe the answer is:
chancy
'uncertain' is the definition.
(similar in meaning)
'at church nancy's part' is the wordplay.
I cannot quite see how this works, but
'church' could be 'ch' (common abbreviation) and 'ch' is found in the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for chancy that I've seen before include "Informally uncertain or risky" , "'Dodgy, dicey (6)'" .)