ACROSS
1. <lighthearted> JOVIAL = JO_ VIA L
4. <judge> FIND = "FINED"
8. <disagreement> CONTRETEMPS = CON MPS outside T_R_E_ T_E
9. H _U + GAMBLER* outside B = HUMBLEBRAG <act of false modesty>
12. INTERLACED* = CREDENTIAL <evidence of authority>
14. <swinging> PROMISCUITY = PROM IS U inside CITY
16. <flow slowly> SEEP = ←PEES←
17. <for fun> IN JEST = "INGEST"

DOWN
1. JACK, O←A = JOCK <sportsman>
2. <taking risks> VENTURESOME = (MOUNT EVERES_)*
3/15. <climate control> AIR-CON = CAIRO* N
5. MEMORIALIST* = IMMORTALISE <to confer everlasting fame>
6. DO SAGE = DOSAGE <amount of medicine>
7. <not fidgeting, perhaps> AT REST = (...e)ATRE ST(a...)
10. BIT outside REX = BREXIT <secession>
11. S + COPS outside E = SCOPES <rifle accessories>
13. _X _Y _S _T = XYST <Greek row?>