Precise guards provided evidence (6)
I believe the answer is:
verify
'evidence' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how they can define each other.
'precise guards provided' is the wordplay.
'precise' becomes 'very' (similar in meaning).
'guards' indicates putting letters inside (inserted letters are guarded).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'very' going around 'if' is 'VERIFY'.
Can you help me to learn more?
(Other definitions for verify that I've seen before include "Check the truth of" , "Ascertain truth or accuracy" , "Prove to be true" , "Check to be true" , "Check (facts)" .)