Exhausted in rows, losing a source of enthusiasm (5)
I believe the answer is:
tired
'exhausted' is the definition.
(exhausted can mean very tired)
'in rows losing a source of enthusiasm' is the wordplay.
'in rows' becomes 'tiered' (laid out in tiers or rows).
'losing' suggests deleting specific letters.
'a source' becomes 'deer' (I have seen 'Source of venison' mean 'deer' so perhaps 'source' could also mean 'deer').
'of enthusiasm' becomes 'red' (I can't justify this - if you can you should give a lot more credence to this answer).
'tiered' with 'deer' taken out is 'ti'.
'ti'+'red'='TIRED'
Can you help me to learn more?
(Other definitions for tired that I've seen before include "Drowsy" , "Weary, fatigued" , "In need of sleep" , "Whacked" , "Are American cars so" .)