but在此是only的意思，for表示持续时间，即to be silly的持续时间。If (it was) only for one afternoon. 省略的主语it即指to be silly. 以下是网上的原文：

Hundreds of Londoners headed down to the Underground on Sunday afternoon, stripped down to their underwear and travelled around a bit, trying to look as though nothing unusual was going on.

This was the Official No Trousers Tube Ride, an annual event with no point other than injecting a little levity into the bleak midwinter. No deep meaning, no bigger motive. The only goal was to be silly, if but for one afternoon.