During study, trainee's first recess (6)
I believe the answer is:
Here is my best explanation:
'recess' is the definition.
(I have seen 'Deep recess ' mean 'indent' so perhaps 'recess' could also mean 'indent')
'during study trainee's first' is the wordplay.
'during' becomes 'in' (I've seen this in other clues).
'study' becomes 'den' (term for a study or office).
'trainee's first' becomes 't' (1st letter of 'trainee').