One probes a part of body most painful (7)
I believe the answer is:
achiest
'most painful' is the definition.
('achy' can be similar in meaning to 'painful')
'one probes a part of body' is the wordplay.
'one' becomes 'i' (Roman numeral).
'probes' indicates putting letters inside.
'part of body' becomes 'chest' ('chest' is part of 'body'**).
'a'+'chest'='achest'
'i' put inside 'achest' is 'ACHIEST'.
(Other definitions for achiest that I've seen before include "suffering most pain" , "most painful" .)