Lid came off fitness test (7)
I believe the answer is:
medical
'fitness test' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'lid came off' is the wordplay.
'off' indicates anagramming the letters.
'lid'+'came'='lidcame'
'lidcame' is an anagram of 'MEDICAL'.
Can you help me to learn more?
(Other definitions for medical that I've seen before include "Doctor's test" , "Declaim (anag) -- health check" , "Relating to doctoring" , "Thorough health check" , "Of the healing sciences" .)