Having a hearty appetite no good, getting very thin (5)
I believe the answer is:
'very thin' is the definition.
(I've seen this before)
'having a hearty appetite no good' is the wordplay.
'having a hearty appetite' becomes 'greedy'.
'no' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'greedy' with 'g' taken out is 'REEDY'.
'getting' is the link.
(Other definitions for reedy that I've seen before include "Like oboe's sound?", "Sounding like eg oboe", "edge of some river-banks", "(Of sound) thin in tone", "Like some pipes".)