Vague exercise is what one might do with fat fingers (7)
I believe the answer is:
mistype
'is what one might do with fat fingers' is the definition.
I can't tell whether this definition defines the answer.
'vague exercise' is the wordplay.
'vague' becomes 'misty' (I've seen this in another clue).
'exercise' becomes 'pe' (Physical Education).
'misty'+'pe'='MISTYPE'
Can you help me to learn more?
(Other definitions for mistype that I've seen before include "Select wrong character" , "Write incorrectly (on a keyboard)" , "Use the wrong keyboard keys" , "Make an error in transcription" , "Hit the wrong key" .)