Oddly, Mary plays in yard when it's wet (5,3)
I believe the answer is:
rainy day
'it's wet' is the definition.
(thesaurus)
'oddly mary plays in yard' is the wordplay.
I cannot really see how this works, but
'in' is present in the answer.
an anagram of 'yard' is 'rayd' which is present in the remaining letters.
The remaining letters 'ay' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'when' acts as a link.
Can you help me to learn more?
(Other definitions for rainy day that I've seen before include "When savings may be needed" , "Save for this future time" , "One saves up for it" , "One should save money for a . . . . . . . . - may need it to buy umbrella" , "Time of financial need" .)