The most common and most likely answer to this crossword clue is the 6 plus 8 letter word PYJAMA TROUSERS
I've seen this crossword clue in The Times
P
Y
J
A
M
A
T
R
O
U
S
E
R
S
Buddy explains!
The straight part
We know that "Pair one assumes heading for the sack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pair one assumes heading for the sack = pyjama trousers.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.