Attentive guard taken short (7)
I believe the answer is:
'attentive' is the definition.
(being mindful of means being attentive to)
'guard taken short' is the wordplay.
'guard' becomes 'mind' (both can mean to look after).
'taken' becomes 'full' (both can mean occupied or in use).
'short' means to remove the last letter.
'full' with its last letter removed is 'ful'.
(Other definitions for mindful that I've seen before include "Taking care" , "Aware and cognizant" , "Aware (of something)" , "Conscious (of); remembering" , "Taking heed" .)