Having changed top, feel unable to relax (5)

I believe the answer is:

tense

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'unable to relax' is the definition.
(I've seen this before)

'having changed top feel' is the wordplay.
'having changed' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'top' means to remove the first letter.
'feel' becomes 'sense' (synonyms).
'sense' with its first letter taken away is 'ense'.
't'+'ense'='TENSE'

Can you help me to learn more?

(Other definitions for tense that I've seen before include "On edge" , "perfect, perhaps" , "Maybe future" , "fraught" , "It may be perfect" .)

I've seen this clue in The Times and The Sunday Times.

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