Mark crossing United States to find bear (7)
I believe the answer is:
sustain
'bear' is the definition.
(I've seen this before)
'mark crossing united states' is the wordplay.
'mark' becomes 'stain' (synonyms).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'united states' becomes 'US' (abbreviation for the United States).
'stain' placed around 'us' is 'SUSTAIN'.
'to find' acts as a link.
(Other definitions for sustain that I've seen before include "Prolong, support" , "Strengthen, support" , "Strengthen physically" , "Support, keep going" , "Keep alive" .)