Agreed to tell them it was full up (4)
I believe the answer is:
pact
'agreed to tell them' is the definition.
I can't judge whether this definition defines the answer.
'it was full up' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'was full' becomes 'cap' (I can't explain this - if you can you should give a lot more credence to this answer).
'up' says the letters should be written backwards (in down clue: letters go upwards).
't'+'cap'='tcap'
'tcap' written backwards gives 'PACT'.
Can you help me to learn more?
(Other definitions for pact that I've seen before include "deal" , "International treaty" , "'Accord, treaty (4)'" , "Treaty, formal agreement" , "Written agreement between two states" .)