1.
If you nail down something unknown or uncertain, you find out exactly what it is.
It would be useful if you could nail down the source of this tension.
= pin down
PHRASAL VERB : V P n (not pron) , also V n P
2.
If you nail down an agreement, you manage to reach a firm agreement with a definite result.
The Secretary of State and his Russian counterpart met to try to nail down the elusive accord.
PHRASAL VERB : V P n (not pron) , also V n P