Always on the retreat, Heather is moving unsteadily (7)

I believe the answer is:

reeling

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'is moving unsteadily' is the definition.
(reeling means staggering or rocking)

'always on the retreat heather' is the wordplay.
'always on' becomes 'eer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the retreat' is a reversal indicator.
'heather' becomes 'ling' (synonyms).
'eer' in reverse letter order is 'ree'.
'ree'+'ling'='REELING'

Can you help me to learn more?

(Other definitions for reeling that I've seen before include "Staggering around" , "Dazed" , "Punchy" , "Dancing; staggering" , "groggy" .)

I've seen this clue in The Times.

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