Clue: Toper wretchedly had no more to drink – does it prevent full flowering? (11)
Answer: PROTERANDRY
Explanation: Definition: does it prevent full flowering?
***(toper)** [anag:*wretchedly*] +** RAN DRY** ("had no more to drink") 
*I'm no botanist, so I'll quote directly from Chambers under "protandry" – in a flower, opening of the anthers before the stigmas can receive pollen, so preventing self-pollination