What acts as a connection is time — hours add up (7)
I believe the answer is:
isthmus
'what acts as a connection' is the definition.
I can't tell whether this defines the answer.
'is time hours add up' is the wordplay.
'time' becomes 't' (abbreviation).
'hours' becomes 'h'.
'add' becomes 'sum' (synonyms).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'sum' written backwards gives 'mus'.
'is'+'t'+'h'+'mus'='ISTHMUS'
Can you help me to learn more?
(Other definitions for isthmus that I've seen before include "Narrow strip of land connecting two large masses" , "Narrow strip of land in sea" , "Strip of land flanked by sea" , "Narrow neck of land joining two larger portions" , "Neck of land connecting larger areas" .)