I was wondering what 一応 would mean in this context. always had trouble grasping the concept of this word.
一応 means something is not meeting a high standard, but should largely meet the minimum requirements. Similarly, it's used when something barely manages to qualify as something.
It's also used to 謙遜. For example:
一応社長やらせてもらってます to appear less arrogant compared to just saying
Conversely, it can also be associated with indicating that someone has a higher social status (like in this specific case). Here, the 先輩 describes whatever was done for the 後輩 was "一応". This subtly reinforces the relationship between the two.
If the 後輩 would use 「一応」to the 先輩 instead, it shows they are close or have almost equal standing to each other. If not, it would sound rude. E.g. if the 後輩, who just joined the club said 「楽器一応拭いときました」, then it would be rude and 生意気.