Appear to understand spy chief (4)
I believe the answer is:
seem
'appear' is the definition.
(synonyms)
'understand spy chief' is the wordplay.
'understand' becomes 'see' (synonyms).
'spy chief' becomes 'm' (M is the head of MI6 in the James Bond series).
'see'+'m'='SEEM'
'to' is the link.
(Other definitions for seem that I've seen before include "Come across as" , "Appear, look" , "Appear, give impression of being" , "Appear to the eye" .)