A ploy: Go, make an expression of regret for failure (7)
I believe the answer is:
apology
'an expression of regret for failure' is the definition.
I can't judge whether this definition defines the answer.
'a ploy go make' is the wordplay.
'make' indicates an anagram.
'ploy'+'go'='ploygo'
'ploygo' with letters rearranged gives 'pology'.
'a'+'pology'='APOLOGY'
Can you help me to learn more?
(Other definitions for apology that I've seen before include "Saying sorry; poor specimen" , "Excuse" , "Expression of regret, very sorry" , "Regretful acknowledgement of a fault" , "Explanation for a fault" .)