Best satisfied when they fall sharply (7)
I believe the answer is:
plummet
'fall sharply' is the definition.
(I've seen this before)
'best satisfied' is the wordplay.
'best' becomes 'plum' (informal term for best or top-class).
'satisfied' becomes 'met' (as in 'met the conditions of his contract').
'plum'+'met'='PLUMMET'
'when they' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for plummet that I've seen before include "Fall at high speed" , "Nose-dive" , "Drop at speed" , "fall sharply" , "Fall rapidly" .)