Retain internal longing as a measure of one's involvement? (4-4)
I believe the answer is:
knee-deep
'involvement?' is the definition.
(I have seen 'big involvement' mean 'knee deep' so perhaps 'involvement' could also mean 'knee deep')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for knee-deep that I've seen before include "Deeply involved (in something)" , "Heavily involved - up to leg joints?" , "Halfway up one's leg" , "Submerged to one's patella" , "Coming half way up the leg" .)