Growth of pleasure - well, that's endless (6)
I believe the answer is:
fungus
'growth' is the definition.
(thesaurus)
'pleasure well that's endless' is the wordplay.
'pleasure' becomes 'fun' (I've seen this before).
'well' becomes 'gu' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that's' becomes 'sc'.
'endless' means to remove the last letter.
'sc' with its last letter removed is 's'.
'fun'+'gu'+'s'='FUNGUS'
'of' is the link.
Can you help me to learn more?
(Other definitions for fungus that I've seen before include "Organism such as a mushroom" , "'Mushroom or toadstool, say (6)'" , "'Lichen, say (6)'" , "A mushroom, toadstool or mould" , "Plant like mushroom or toadstool" .)