Achilles to an extent unwell (3)
I believe the answer is:
ill
'unwell' is the definition.
(similar in meaning)
'achilles to an extent' is the wordplay.
'to an extent' indicates a hidden word.
'ILL' can be found hidden inside 'achilles'.
(Other definitions for ill that I've seen before include "weak" , "Out of sorts" , "trouble" , "Green round the gills" , "Not in good health" .)