Taciturn stoic regularly going in late (8)
I believe the answer is:
'taciturn' is the definition.
(similar in meaning)
'stoic regularly going in late' is the wordplay.
'regularly going' means one should take alternating letters (letters removed at regular intervals).
'in' indicates putting letters inside.
'late' becomes 'recent' (similar in meaning).
The alternate letters of 'stoic' are 'ti'.
'ti' placed into 'recent' is 'RETICENT'.
(Other definitions for reticent that I've seen before include "Not revealing one's thoughts readily" , "Reserved, loath to talk" , "Disinclined to speak" , "Quiet" , "Reserved, shy" .)