Check viscous fluid including tar, possibly (8)
I believe the answer is:
restrain
'check' is the definition.
(I know that check is a more specific form of the action restrain)
'viscous fluid including tar possibly' is the wordplay.
'viscous fluid' becomes 'resin' (I can't justify this - if you can you should give a lot more credence to this answer).
'including' is an insertion indicator.
'possibly' indicates anagramming the letters.
'tar' is an anagram of 'tra'.
'resin' enclosing 'tra' is 'RESTRAIN'.
Can you help me to learn more?
(Other definitions for restrain that I've seen before include "Keep back" , "use handcuffs?" , "bar" , "Control - check" , "Manacle" .)