Only few will be admitted when you find the key (5)
I believe the answer is:
solve
'key' is the definition.
The definition and answer can be both related to thinking as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?
'only few will be admitted when you find' is the wordplay.
I cannot really see how this works, but
'few' could be 'v' and 'v' is present in the answer.
'only' could be 'sole' (synonyms) and 'sole' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'the' is the link.
Can you help me to learn more?
(Other definitions for solve that I've seen before include "Complete crossword" , "Find the answer to this clue, for instance" , "do crosswords" , "Settle" , "Find the solution to puzzle or problem" .)