Having a position when in sack (5)
I believe the answer is:
based
'having a position' is the definition.
(being based in a particular place)
'when in sack' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'in' means one lot of letters goes inside another.
'sack' becomes 'bed' ('in the sack' means 'in bed').
'as' going within 'bed' is 'BASED'.
(Other definitions for based that I've seen before include "Stationed (in a place)" , "foundation" , "Founded, grounded" , "Positioned" , "Located" .)