Evasive firm, untrustworthy ultimately (3)
I believe the answer is:
Here is my best explanation:
'evasive' is the definition.
'firm untrustworthy ultimately' is the subsidiary indication.
'firm' becomes 'co'.
'ultimately' indicates one should take the final letters.
The final letter of 'untrustworthy' is 'y'.
(Other definitions for coy that I've seen before include "Affectedly modest or shy", "Pretending to be shy or bashful", "Modest", "Bashful", "Shy", "Pretending shyness", "Demure, like Marvell's mistress", "Pretending modesty" and "Pretending to be shy or embarrassed".)