German fellow's regimen accepted with bit of hesitation (6)
I believe the answer is:
dieter
'german fellow's' is the definition.
(I've seen this before)
'regimen accepted with bit of hesitation' is the wordplay.
'regimen' becomes 'diet' (I've seen this before).
'accepted with' means one lot of letters go next to another.
'bit of hesitation' becomes 'er' (what a hesitating person might say).
'diet'+'er'='DIETER'
(Other definitions for dieter that I've seen before include "German fellow" , "fellow who'll eat less?" , "German boy" , "one trying to lose weight" , "German male" .)