Cautious about starting early when tired (5)
I believe the answer is:
weary
'tired' is the definition.
('weary' can be similar in meaning to 'tired')
'cautious about starting early' is the wordplay.
'cautious' becomes 'wary' ('wary' can be similar in meaning to 'cautious').
'about' indicates putting letters inside.
'starting' indicates taking the first letters.
The initial letter of 'early' is 'e'.
'wary' placed around 'e' is 'WEARY'.
'when' acts as a link.
(Other definitions for weary that I've seen before include "Tired - bored" , "Worn-out" , "Exhausted" , "spent" , "Flagging" .)