Mark gives thanks to Gert, possibly (6)
I believe the answer is:
target
'mark' is the definition.
(I know that mark can be written as target)
'thanks to gert possibly' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'possibly' indicates an anagram.
'gert' anagrammed gives 'rget'.
'ta'+'rget'='TARGET'
'gives' is the link.
(Other definitions for target that I've seen before include "desired objective" , "Aimpoint" , "Something to aim for" , "'Aim, goal (6)'" , "Goal; victim" .)