Playing guitar — extremely timely tip (8)
I believe the answer is:
gratuity
'tip' is the definition.
(I know that tip can be written as gratuity)
'playing guitar extremely timely' is the wordplay.
'playing' indicates an anagram.
'extremely' suggests removing the centre (only the extremes of the word are used).
'timely' with its middle taken out is 'ty'.
'guitar' is an anagram of 'gratui'.
'gratui'+'ty'='GRATUITY'
(Other definitions for gratuity that I've seen before include "Tip (to waiter)" , "present" .)