Facial expression as muscle tightens to begin with (6)
I believe the answer is:
aspect
'facial expression' is the definition.
(I've seen this before)
'as muscle tightens to begin with' is the wordplay.
'muscle' becomes 'pec' (pectoral muscle).
'to begin with' indicates taking the first letters.
The initial letter of 'tightens' is 't'.
'as'+'pec'+'t'='ASPECT'
(Other definitions for aspect that I've seen before include "'Appearance, outlook (6)'" , "'View, perspective (6)'" , "Vista, panorama" , "Characteristic to be considered" , "Viewpoint or view" .)