EF-2016: In the frame of the European Fellowship is it possible to have secondment in the TC (for example: Singapore)?

Secondment is possible only to another organisation in EU/AC. Short visits are possible in any country of the world.
Having set the legal context, and to answer to your specific question about short visits and field work: this would require a case-by-case analysis to be undertaken by the Project Officer in charge of the management of each project. I would therefore recommend that the beneficiary contacts its Project Officer in such case.