Keep thanks in check (6)
I believe the answer is:
retain
'keep' is the definition.
(synonyms)
'thanks in check' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'in' is an insertion indicator.
'check' becomes 'rein' (both can mean to control or limit).
'ta' going inside 'rein' is 'RETAIN'.
(Other definitions for retain that I've seen before include "'Hold, keep back (6)'" , "Hold in position" , "Not relinquish" , "Hold onto" , "Keep in Tirane" .)