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 "Bashful", "Pretending shyness", "not forthcoming", "Pretending modesty", "Reserved", "reluctant to divulge information", "Group of soldiers", "Pretending to be shy or bashful", "Demure, like Marvell's mistress" and "Modest".)