Principal reason Mecca empty in season (10)
I believe the answer is:
mainspring
'principal reason' is the definition.
'mainspring' can be an answer for 'reason' (thesaurus). I'm not sure about the 'principal' bit.
'mecca empty in season' is the wordplay.
'empty' suggests removing the centre.
'season' becomes 'spring' (spring is a season).
'mecca' with its centre removed is 'ma'.
'ma'+'in'+'spring'='MAINSPRING'
Can you help me to learn more?
(Other definitions for mainspring that I've seen before include "Chief motive" , "motivating force" , "Pining rams (anag.)" , "essential to watch" , "Principal agent of motivation" .)