Mat somehow gets left outside in combat (8)
I believe the answer is:
struggle
'combat' is the definition.
(thesaurus)
'mat somehow gets left outside' is the wordplay.
'mat' becomes 'rug' (I've seen this before).
'somehow' indicates anagramming the letters.
'left' becomes 'l' (common abbreviation).
'outside' indicates putting letters inside.
'gets'+'l'='getsl'
'getsl' is an anagram of 'stgle'.
'rug' placed inside 'stgle' is 'STRUGGLE'.
'in' is the link.
(Other definitions for struggle that I've seen before include "Contend; very hard task" , "Strive, contend" , "Proceed with great effort" , "Progress with difficulty" , "Conflict involving effort" .)