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 really see how this works, but
'church' could be 'ch' (common abbreviation) and 'ch' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a 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)'" .)