The most common and most likely answer to this crossword clue is the 7 letter word MILLDAM
I've seen this crossword clue in Financial Times
M
I
L
L
D
A
M
More clues leading to the result MILLDAM
Buddy explains!

The straight part
We know that "Retainer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case retainer = milldam.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MILLDAM mean?
MILLDAM noun- A dam constructed across a river or stream to raise the water level so that it can turn a millwheel; also, the millpond so created.