Pin-up I do love, endlessly (4)
I believe the answer is:
idol
'pin-up' is the definition.
(I've seen this before)
'i do love endlessly' is the wordplay.
'do love endlessly' becomes 'dol' (I am not sure about this - if you are sure you should believe this answer much more).
'i'+'dol'='IDOL'
Can you help me to learn more?
(Other definitions for idol that I've seen before include "something to be revered" , "Graven image or greatly admired person" , "pop star" , "Something worshipped" , "One much admired" .)