Variable cloud whenever retreating in time for military exercises (5,3)
I believe the answer is:
field day
'time for military exercises' is the definition.
I can't judge whether this defines the answer.
'variable cloud whenever retreating' is the wordplay.
'variable' becomes 'y' (algebra).
'cloud' becomes 'addle' (I can't explain this - if you can you should give a lot more credence to this answer).
'whenever' becomes 'if' (I've seen this in other clues).
'retreating' says the letters should be written in reverse.
'y'+'addle'+'if'='yaddleif'
'yaddleif' written backwards gives 'FIELD DAY'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for field day that I've seen before include "Military exercise - time when everything goes easily" , "productive time" , "short scientific expedition" , "Successful, exciting period" .)