They gather firm repeatedly left escort adrift (10)
I believe the answer is:
Here is my best explanation:
'they gather' is the definition.
'firm repeatedly left escort adrift' is the subsidiary indication.
'firm' becomes 'co'.
'repeatedly' says to double the letters.
'left' becomes 'l' (common abbreviation**).
'adrift' indicates anagramming the letters.
'l' duplicated is 'll'.
'escort' with letters rearranged gives 'ectors'.
(Another definition for collectors that I've seen is "They gather".)