Scot with a bit of stomach ache? (3)
I believe the answer is:
mac
'scot' is the definition.
(many Scottish surnames begin with Mac)
'bit of stomach ache?' is the wordplay.
'bit of' indicates the answer is hidden within the clue.
'MAC' can be found hidden inside 'stomachache'.
'with a' acts as a link.
(Other definitions for mac that I've seen before include "PC" , "laptop?" , "A Gaelic son, often in name" , "Waterproof raincoat, in short" , "It may help you keep dry" .)