Elasticity of university head's clothing (6)
I believe the answer is:
bounce
'elasticity' is the definition.
(bounce is a kind of elasticity)
'university head's clothing' is the wordplay.
'university' becomes 'U' (abbreviation for university).
'head' becomes 'bonce' (informal term for a person's head).
'clothing' indicates putting letters inside (some letters cover up or clothe others).
'u' inserted within 'bonce' is 'BOUNCE'.
'of' acts as a link.
(Other definitions for bounce that I've seen before include "Hop like rubber ball" , "What a cheque may do if there are insufficient funds" , "Recoil like rubber ball" , "Springiness" , "Move energetically; rebound" .)