Oddly flat appeal with high confidence (5)
I believe the answer is:
'confidence' is the definition.
(I've seen this before)
'oddly flat appeal with high' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'appeal' becomes 'it'.
'with' means one lot of letters go next to another.
'high' becomes 'h' (this could be a standard abbreviation I've not previously seen).
The alternating letters of 'flat' are 'fa'.
Can you help me to learn more?
(Other definitions for faith that I've seen before include "Total trust", "Unshakeable belief", "Staunch trust", "religion", "Strong belief in a supernatural power".)