It determines the regularity of beatings (11)
I believe the answer is:
stethoscope
'it determines the regularity of beatings' is the definition.
The definition suggests a singular noun which matches the answer.
'it determines the regularity of' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
an anagram of 'the' is 'eth' which is within the answer.
'of' could be 'o' and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stethoscope that I've seen before include "Instrument used in listening" , "Doctor's instrument" , "Medical instrument" , "listening device" .)