Pin down late supporter about tense, tense situation (4-5)
I believe the answer is:
nail-biter
'tense situation' is the definition.
(I've seen this before)
'pin down late supporter about tense' is the wordplay.
'pin down' becomes 'nail' (I've seen this before).
'late supporter' becomes 'bier' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'tense' becomes 't' (grammatical abbreviation).
'bier' going around 't' is 'biter'.
'nail'+'biter'='NAIL-BITER'
Can you help me to learn more?
(Another definition for nail-biter that I've seen is " One chewing fingers".)