Extending across river when current is accommodating (7)
I believe the answer is:
astride
'extending across' is the definition.
(to be astride something is to extend across it)
'river when current is accommodating' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'current' becomes 'tide' (sea currents).
'is accommodating' is an insertion indicator (inserted letters are held or accommodated).
'as'+'tide'='astide'
'r' going within 'astide' is 'ASTRIDE'.
(Other definitions for astride that I've seen before include "Not side saddle" , "With legs apart, emitting tirades" , "With one leg on either side, as sitting on horse" , "On top of" , "Straddling (horse)" .)