Crossword Genius

Advance or keep on retreating? (8)

Ross

I believe the answer is:

retainer

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'advance' is the definition.
(I've seen this before)

'keep on retreating?' is the wordplay.
'keep on' becomes 'retain' (synonyms).
'retreating?' becomes 'er' (I can't justify this - if you can you should believe this answer much more).
'retain'+'er'='RETAINER'

'or' is the link.

Can you help me to learn more?

(Other definitions for retainer that I've seen before include "Family servant; barrister's fee" , "Fee paid to secure services" , "Eritrean (anag)" , "Old servant - preliminary fee" , "Fee paid to book professional services" .)

I've seen this clue in The Sunday Times.
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