A large enthusiast is everywhere (3,4)
I believe the answer is:
all over
'is everywhere' is the definition.
(I've seen this before)
'a large enthusiast' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'enthusiast' becomes 'lover' (someone who loves something).
'a'+'l'+'lover'='ALL-OVER'
(Other definitions for all over that I've seen before include "very characteristically" , "Obsessed with" , "Finished" , "Excessively attentive" , "typically" .)