Escort agency below adult clubs (9)
I believe the answer is:
accompany
'escort' is the definition.
(I've seen this before)
'agency below adult clubs' is the wordplay.
'agency' becomes 'company' (both can mean an organisation).
'below' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'adult' becomes 'a' (abbreviation).
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'a'+'c'='ac'
'company' after 'ac' is 'ACCOMPANY'.
(Other definitions for accompany that I've seen before include "Travel along with" , "Unbelief" , "Escort" , "Go with as a companion" .)