Extravagantly praising man, lacking a motive (10)
I believe the answer is:
mainspring
'motive' is the definition.
(I have seen 'Chief motive' mean 'mainspring' so perhaps 'motive' could also mean 'mainspring')
'extravagantly praising man lacking a' is the wordplay.
'extravagantly' indicates anagramming the letters (an unusual or extravagant version of the letters).
'lacking' indicates named letters should be taken away.
'praising'+'man'='praisingman'
'praisingman' with 'a' taken out is 'prisingman'.
'prisingman' is an anagram of 'MAINSPRING'.
(Other definitions for mainspring that I've seen before include "Chief motive" , "Pining rams (anag.)" , "motivating force" , "It motivates" , "Principal agent of motivation" .)