Check whether most suitable to go outside (6)
I believe the answer is:
verify
'check' is the definition.
(to verify is to check something is true)
'whether most suitable to go outside' is the wordplay.
'whether' becomes 'if'.
'most suitable' becomes 'very' (as in 'the very thing').
'to go outside' is an insertion indicator.
'if' placed inside 'very' is 'VERIFY'.
(Other definitions for verify that I've seen before include "Check to be true" , "Establish truth or accuracy of" , "Ascertain truth or accuracy" , "Check (facts)" , "Support" .)