Old mischievous gent, hot in sack, going around hotel (4,2,3,5)
I believe the answer is:
long in the tooth
'old' is the definition.
(I've seen this before)
'gent hot in sack going around hotel' is the wordplay.
'sack' indicates an anagram (sack can mean destroy or spoil).
'going' becomes 'loot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'around' means one lot of letters goes inside another.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'gent'+'hot'+'in'='genthotin'
'genthotin' with letters rearranged gives 'ngintheto'.
'ngintheto' going into 'loot' is 'longinthetoot'.
'longinthetoot'+'h'='LONG IN THE TOOTH'
'mischievous' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for long in the tooth that I've seen before include "Getting on a bit" , "like smilodon?" , "Rather old" , "over the hill?" , "Elderly" .)