Stretched leaders of one Nazi group cracking up (9)
I believe the answer is:
elongated
'stretched' is the definition.
('elongate' can be a synonym of 'stretch')
'leaders of one nazi group cracking up' is the wordplay.
'leaders of' says to take the initial letters.
'cracking' means one lot of letters goes inside another (inserted letters go into a gap or crack).
'up' becomes 'elated' (both can mean very happy).
The first letters of 'one nazi group' is 'ong'.
'ong' put into 'elated' is 'ELONGATED'.
(Other definitions for elongated that I've seen before include "Put on stretcher" , "going further" , "Stretched" , "Dangle toe (anag)" , "Go al dente to be lengthened or prolonged" .)