Understand cannula must be emptied - start to turn both taps round periodically (5,2)
I believe the answer is:
catch on
'understand' is the definition.
(I've seen this in another clue)
'cannula must be emptied start to turn both taps round periodically' is the wordplay.
I cannot really see how this works, but
'turn' could be 't' (abbreviation in Oxford Dictionary of Abbreviations) and 't' is found in the answer.
'taps' could be 'ch' (cold and hot taps) and 'ch' is located in the answer.
'round' could be 'o' (round can mean a round object) and 'o' is located in the answer.
The remaining letters 'can' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for catch on that I've seen before include "twig" , "Informally, become popular or come to understand" , "Understand - become fashionable" , "Get it" .)