Establish the extent of moans being expressed (4)
I believe the answer is:
size
'establish the extent' is the definition.
I can't tell whether this defines the answer.
'moans being expressed' is the wordplay.
'moans' becomes 'sighs' (to sigh is to moan or sound dejected).
'being expressed' indicates a 'sounds like' (homophone) clue.
'sighs' is a homophone of 'SIZE'.
'of' is the link.
Can you help me to learn more?
(Other definitions for size that I've seen before include "Volume" , "gelatinous mixture" , "Dimension" , "Measure" , "Measurement or extent of something" .)