Definition: "possible reason for lack of appetite"

This is a charade - multiple word parts are placed together in sequence to build the final answer.

Look for a 3-letter word for 'agent', then a 2-word phrase meaning 'revealed' that goes around a single letter representing 'lunchtime'. The question mark after 'lunchtime' suggests this part might be cryptic or unconventional.

Answer: REPLETION | REP (agent) + LET ON (revealed) around I (one o'clock = lunchtime). The state of being full, which would cause lack of appetite.