Not able to remove an attachment? (5)

I believe the answer is:

unfit

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'to remove an attachment?' is the definition.
I can't tell whether this defines the answer.

'not able' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'able' becomes 'fit' (similar in meaning).
'un'+'fit'='UNFIT'

Can you help me to learn more?

(Other definitions for unfit that I've seen before include "Inapt" , "Not healthy" , "not worthy" , "Incompetent" , "Not in good physical condition, or not suitable" .)

I've seen this clue in the Evening Standard.

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