Spot invaded by American bear (7)
I believe the answer is:
sustain
'bear' is the definition.
(thesaurus)
'spot invaded by american' is the wordplay.
'spot' becomes 'stain' ('stain' can be a synonym of 'spot'**).
'invaded by' means one lot of letters goes inside another.
'american' becomes 'us'.
'stain' placed around 'us' is 'SUSTAIN'.
(Other definitions for sustain that I've seen before include "Keep alive" , "Maintain" , "Strengthen, support" , "Give food to" , "Keep up; hold for a long time" .)