Expert in mid-August to feel endless regret (4)
I believe the answer is:
guru
'expert' is the definition.
(I've seen this before)
'mid-august to feel endless regret' is the wordplay.
'mid' means to look at the middle letters (mid- can mean the middle of something).
'to feel' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'endless' says to take the centre.
'regret' becomes 'rue' (to rue something is to regret it).
The middle of 'august' is 'gu'.
The centre of 'rue' is 'u'.
'gu'+'r'+'u'='GURU'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for guru that I've seen before include "Master, mentor" , "Recognised expert in some field" , "Recognised leader in some field" , "Religious leader and spiritual teacher" , "Authoritative source" .)