During study, trainee’s first recess (6)
I believe the answer is:
indent
'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' (e.g. during February means in February).
'study' becomes 'den' (term for a study or office).
'trainee's first' becomes 't' (1st letter of 'trainee').
'in'+'den'+'t'='INDENT'
(Other definitions for indent that I've seen before include "Make a requisition (for)" , "Start (line) in from edge of text" , "Form notches in" , "Deep recess; official requisition" , "Notch on edge" .)